At least five prominent British Islamists linked to the same extremist group frequented by the man suspected of being the new “Jihadi John” executioner left the UK in the last 20 months despite being on bail or subject to travel bans.

Records show that the five men, three of whom have since been killed in drone or air strikes in Syria and Pakistan, all succeeded in evading exit checks despite being on watch lists or having surrendered their passports to the authorities.

The ability of the known extremists to flee under the noses of police and the security services will increase the pressure on the Government to tighten monitoring of individuals known to be at risk of travelling to Syria after Muslim convert Siddhartha Dhar slipped out of Britain to join Isis while on police bail for suspected terror offences.

Father-of-four Dhar, who now goes by the name of Abu Rumaysah, had been ordered by police to surrender his passport hours before he boarded a bus to Paris with his family in September 2014. Scotland Yard remained unaware of his whereabouts until at least a month later.

The British-Indian is now strongly suspected of being the English-accented gunman who oversees the execution of five alleged British spies in an Isis propaganda video released on Sunday.

Labour MP Keith Vaz, chairman of the influential Commons’ Home Affairs select committee, described the cases as a “glaring security failure” and vowed to raise the issue with Prime Minister David Cameron.

Three of the five men have been killed in drone or air strikes in Syria and Pakistan. File photo. (Getty) (Getty Images)

He said: “The number of cases where extremists and individuals known to the authorities have left the UK after being told to hand in their passports, or who are subject to a travel ban, is astounding.

“There is no point in the government trumpeting their ‘achievement’ of 100% exit checks when the people of greatest concern are able to leave with ease.”

Who is the man in IS video?

The five men were all linked to Al Muhajiroun, the banned Islamist group whose rallies were frequently attended by Dhar, 32, and other associated organisations, including Muslims Against Crusaders, which was banned in 2011 after its members publicly burned poppies on Armistice Day.

The five who left Britain Abu Rahin Aziz, 32, left UK March 2014: Luton-based Aziz was a prominent activist in the Muslims Against Crusaders group, which burned poppies on Armistice Day in 2010. After arriving in Syria he tweeted about his friendship with fellow Briton Siddhartha Dhar, expressing the hope that the pair would one day pose with the severed head of a British military pilot. Aziz was killed in a drone strike last July.

Luton-based Aziz was a prominent activist in the Muslims Against Crusaders group, which burned poppies on Armistice Day in 2010. After arriving in Syria he tweeted about his friendship with fellow Briton Siddhartha Dhar, expressing the hope that the pair would one day pose with the severed head of a British military pilot. Aziz was killed in a drone strike last July. Junaid Hussain, 21, left March 2014: The former student and hacker from Birmingham was on police bail for violent disorder when he left for Syria. He was killed last August in a US air strike after allegedly directing a series of Isis plots against western targets.

The former student and hacker from Birmingham was on police bail for violent disorder when he left for Syria. He was killed last August in a US air strike after allegedly directing a series of Isis plots against western targets. Mirza Tariq Ali, 39, left April 2014: The Pakistan-born surgeon had been the head of the Sharia4Pakistan group, which caused controversy in 2012 when it claimed to have issued a fatwa against Malala Yousafzi. Ali fled Britain while on bail to lead a Taliban splinter group and was killed in a drone strike.

The Pakistan-born surgeon had been the head of the Sharia4Pakistan group, which caused controversy in 2012 when it claimed to have issued a fatwa against Malala Yousafzi. Ali fled Britain while on bail to lead a Taliban splinter group and was killed in a drone strike. Trevor Brooks, 40, left November 2015: Also known as Abu Izzadeen, Muslim preacher Brooks was arrested in Hungary after leaving Britain in breach of a travel ban imposed for a conviction for inciting terrorism. Prosecutors said it had seemed he never intended to return to the UK.

Also known as Abu Izzadeen, Muslim preacher Brooks was arrested in Hungary after leaving Britain in breach of a travel ban imposed for a conviction for inciting terrorism. Prosecutors said it had seemed he never intended to return to the UK. Simon Keeler, 44, left November 2015: Also known as Sulayman Keeler, the convert was travelling alongside Brooks when the pair were arrested. He reportedly presented his driving licence as identification when challenged by Hungarian authorities. The Briton was cleared in July last year of attempting to leave the UK on a false passport to join Isis.

The Independent understands that at least two of the men travelled to join Isis in Syria using their own passports, including Luton resident Abu Rahin Aziz, 32, a leading activist in Muslims Against Crusaders who later met up with Dhar, a former bouncy castle salesman, in the Isis nerve centre of Raqqa.

Aziz, who had been facing trial for a violent assault, posted a picture last February of the pair posing beside a crashed fighter jet. A message alongside expressed the hope that the next time the pair met they would be holding the severed head of a British pilot.

Including Dhar, four of the men were able to leave Britain despite being on police or court bail for terror or violence-related offences and known to the security services as prominent figures in Islamist fringe groups. The remaining two - Simon Keeler and Trevor Brooks, Muslim converts with convictions related to terrorist fundraising - were arrested in Hungary in November having left Britain despite both being under travel bans.

Brooks, who along with Keeler may have been trying to reach Syria, had had his passport confiscated last April on national security grounds and was travelling using his driving licence.

Anti-extremist group Hope Not Hate, which revealed the five cases, said the number of prominent radicals succeeding in leaving the country raised serious questions about the state of Britain’s counter-terrorism controls.

Timeline: The emergence of Isis Show all 40 1 /40 Timeline: The emergence of Isis Timeline: The emergence of Isis 2000 Abu Musab al-Zarqawi (pictured here) forms an al-Qaeda splinter group in Iraq, al-Qa’eda in Iraq. Its brutality from the beginning alienates Iraqis and many al-Qaeda leaders. Timeline: The emergence of Isis 2006 Al-Zarqawi is killed in a U.S. strike. Al-Zarqawi’s successor, Abu Ayyub al-Masri, announces the creation of the Islamic State in Iraq (ISI). Reuters Timeline: The emergence of Isis 2009 Still al-Qaeda-linked ISI claims responsibility for suicide bombings that killed 155 in Baghdad, as well as attacks in August and October killing 240, as President Obama announces troop withdrawal from Iraq in March. Getty Images Timeline: The emergence of Isis 2010 Abu Bakr al-Baghdadi becomes head of ISI, at lowest ebb of Islamist militancy in Iraq, which sees last U.S. combat brigade depart. Timeline: The emergence of Isis 2012 In Syria, protests (pictured here starting in Daree) have morphed into what president Assad labelled a “real war” with emergence of a coalition of forces opposed to Assad’s regime. Syria group Jabhat al-Nusra are among rebel groups who refuse to join, denouncing it as a “conspiracy”. Bombings targeting Shia areas, killing more than 500 people, spark fears of new sectarian conflict. Sunni Muslims stage protests across country against what they see as increasingly marginalisation by Shia-led government. AP Timeline: The emergence of Isis 2013 Al-Baghdadi renames ISI as the Islamic State in Iraq and Syria, or Isis, as the group absorbs Syrian al-Nusra, gaining a foothold in Syria. In response, al-Qaeda chief Ayman al-Zawahiri (Bin Laden’s successor) concerned about Isis’ expansion orders that Isis be dissolved and ISI operations should be confined to Iraq. This order is rejected by al-Baghdadi. AFP Timeline: The emergence of Isis 2014 - January Isis fighters capture the Iraqi cities of Fallujah and Ramadi, giving them base to launch slew of attacks further south. AP Timeline: The emergence of Isis 2014 - June Isis declares itself the Caliphate, calling itself Islamic State (IS). The group captures Mosul, Iraq’s second largest city; Tal Afar, just 93 miles from Syrian border; and the central Iraqi city of Tikrit. These advances sent shockwaves around the world. Timeline: The emergence of Isis 2014 - June Around the same time Isis releases a video calling for western Muslims to join the Caliphate and fight, prompting new evaluations of extremists groups social media understanding. Timeline: The emergence of Isis 2014 - June Isis take Baiji oil fields in Iraq - giving them access to huge amounts of possible revenue. EPA Timeline: The emergence of Isis 2014 - August James Foley is executed by the group as concerns grow for second American prisoner, fellow reporter Steven Sotloff. AP Timeline: The emergence of Isis 2014 - August Obama authorises U.S. airstrikes in Iraq, helping to stall Isis’ along with action by Kurdish forces following the deaths of hundreds of Yazidi people on Mount Sinjar. Timeline: The emergence of Isis 2014 - September Isis release video showing Steven Sotloff’s murder prompting Western speculation his executioner is same man who killed Mr Foley. EPA Timeline: The emergence of Isis 2014 - September Obama tells us that America “will hunt down terrorists who threaten our country” EPA Timeline: The emergence of Isis 2014 - September Isis release a video appearing to show David Haines, who was captured by militants in Syria in 2013, wearing an orange jumpsuit and kneeling in the desert while he reads a pre-prepared script. It later shows what appears to be the aid worker's body. Rex Timeline: The emergence of Isis 2014 - September Peshmerga fighters scrabble to hold positions in the Diyala province (a gateway to Baghdad) as Isis fighters continue to advance on Iraqi capital. AFP Timeline: The emergence of Isis 2014 - October Aid worker Alan Henning is killed. Self-imposed media blackout refuses to show images of him in final moments, instead focuses upon humanitarian care. AP Timeline: The emergence of Isis 2014 - October Isis raise their flag in Kobani, which had been strongly defended by Kurdish troops. The victory goes against hopeful western analysis Isis had overextended itself, while alienating much of the Muslim population through the murder of Henning. Victory causes fresh waves of Kurdish refugees arriving in Turkey. Timeline: The emergence of Isis 2014 - November American hostage, who embarced values of Islam, Peter Kassig and 14 Syrian soldiers are shown meeting the same fate as other captives. But intelligence agencies will be poring over the apparently significant discrepancies between this and previous films. Seramedig.org.uk Timeline: The emergence of Isis 2015 - February Isis has released a video revealing the murder by burning to death of a Jordanian pilot held by the group since the end of December 2014. Reuters Timeline: The emergence of Isis 2015 - February Isis militants have released videos which appear to show the beheading of Japanese hostages Haruna Yukawa and Kenji Goto. Timeline: The emergence of Isis 2015 - February American aid worker, Kayla Mueller was the last American hostage known to be held by Isis. She died, according to her captors, in an airstrike by the Jordanian air force on the city of Raqqa in Syria, though US authorities disputed this. AP Timeline: The emergence of Isis 2015 - February Isis militants have posted a gruesome video online in which they force 21 Egyptian Coptic Christian hostages to kneel on a beach in Libya before beheading them. Egypt vowed to avenge the beheading and launched air strikes on Isis positions. AP Timeline: The emergence of Isis 2015 - February The British Isis militant suspected of appearing in videos showing the beheading of Western hostages has been named in reports as Mohammed Emwazi from London. Rex Features Timeline: The emergence of Isis 2015 - March Isis triple suicide attack has killed more than 100 worshippers and hundreds of others were injured after the group members targeted two mosques in the Yemeni capital of Sanaa. AP Timeline: The emergence of Isis 2015 - April Iraqi forces have claimed victory over Isis in battle for Tikrit and raised the flag in the city. EPA/STR Timeline: The emergence of Isis 2015 - April Isis has claimed responsibility for a suicide bomb attack in Afghanistan that killed at least 35 people queuing to collect their wages and injured 100 more. EPA Timeline: The emergence of Isis 2015 - April Isis’ media arm released a 29-minute video purporting to show militants executing Ethiopian Christians captives. The footage bore the extremist group’s al-Furqan media logo and showed the destruction of churches and desecration of religious symbols. A masked fighter made a statement threatening Christians who did not convert to Islam or pay a special tax. Timeline: The emergence of Isis 2015 - May Abu Bakr al-Baghdadi, the leader of Isis has been "incapacitated" by a spinal injuries sustained in a US air strike in Iraq. He is being treated in a hideout by two doctors from Isis’ stronghold of Mosul who are said to be "strong ideological supporters of the group". Timeline: The emergence of Isis 2015 - May Isis has also claimed responsibility for killing 300 of Yazidi captives, including women, children and elderly people in Iraq AP Timeline: The emergence of Isis 2015 - May Isis attack on Prophet Mohamed cartoon contest in Texas was its first action on US soil. Two gunmen were shot and killed after launching the attack at the exhibition. Elton Simpson and Nadir Soofi have been named as the attackers at the Curtis Culwell Centre arena in Garland. Timeline: The emergence of Isis 2015 - May Isis’s deputy leader, Abu Alaa Afri, a former physics teacher who was thought to have taken charge of the deadly terrorist group, has been killed in a US-led coalition airstrike. Timeline: The emergence of Isis 2015 - May US special forces have killed a senior Isis leader named as Abu Sayyaf in an operation aiming to capture him and his wife in Syria. Getty Images Timeline: The emergence of Isis 2015 - May Iran-backed militias are sent to Ramadi by the Iraqi government to fight Isis militants who completed their capture of the city. Government soldiers and civilians were reportedly massacred by extremists as they took control and the army fled. Charred bodies were left littering the city streets as troops clung on to trucks speeding away from the city. Ramadi is the latest government stronghold to fall to the so-called Islamic State, despite air strikes by a US-led international coalition aiming to stop its advance in Iraq and Syria. AFP Timeline: The emergence of Isis 2015 - May Isis rounded up civilians trapped in Palmyra and forced them to watch 20 people being executed in the historic city’s ancient amphitheatre. The Unesco World Heritage site was overrun by militants, threatening the future of 2,000 year-old monuments and ruins. Thousands of Palmyra’s residents fled but many are still living within the city walls, while the UN human rights office in Geneva said it had received reports of Syrian government forces preventing people from leaving until they retreated from the city. Getty Timeline: The emergence of Isis 2015 - May A group of Isis-affiliated fighters have captured a key airport in central Libya. The militants took control of the al-Qardabiya airbase in Sirte after a local militia tasked with defending the facility withdrew from their positions. Affiliates of Isis, already control large parts of Sirte, the birthplace of former Libyan leader Muammar Gaddafi and a former stronghold of his supporters. Timeline: The emergence of Isis 2015 - June The US Air Force has destroyed an Isis stronghold after an extremist let slip their location on social media. According the Air Force Times, General Herbert "Hawk" Carlisle, commander of Air Combat Command, said that Airmen at Hulburt Field, Florida, used images shared by jihadists to track the location of their headquarters before destroying it in an airstrike. Reuters Timeline: The emergence of Isis 2015 - June Kurdish forces captured a key military base in a significant victory in Raqqa as well as town of Tell Abyad. YPG fighters, backed by US-led airstrikes and other rebels, consolidated their gains, when they seized the key town on the Syria-Turkey border. They are now just 30 miles to the north of Raqqa and have cut off a major supply route deep inside Isis-held territory. Ahmet Silk/Getty Timeline: The emergence of Isis 2015 - June Isis has released gruesome footage claiming to show the murder of more than a dozen men by drowning, decapitation and using a rocket-propelled grenade as it seeks to boost morale among its fanatical supporters. Timeline: The emergence of Isis 2015 - June Isis has begun carrying out its threat to destroy structures in the ancient Syrian city of Palmyra, blowing up at least two monuments at the Unesco-protected site as Syrian government troops made advances on the Islamist’s positions. AFP

Nick Lowles, co-ordinator for the organisation, said: “We have a situation where at least four individuals who were on active bail and must have been known to the authorities as a flight risk have managed to leave Britain.

Siddhartha Dhar, circled, at an Al Muhajiroun rally in 2011 (Rex) (Rex Features)

“Given all six were linked to banned organisations like Al Muhajiroun, there must be very serious concerns about the apparent ease with which individuals who wish Britain harm are leaving the country.”

One of the first men to leave Britain was Mirza Tariq Ali, a 39-year-old NHS surgeon who had been due to stand trial at the Old Bailey for violent disorder after he allegedly struck a bystander during an Al Muhajiroun protest.

Despite having surrendered his passport as part of the proceedings, he was able to leave in April 2014 and reach Pakistan, where he became a leader of the country’s Taliban and was killed in a drone strike in March.

Aziz, who was sentenced to 36 months’ imprisonment for stabbing football fan in the head with a pen, is thought to have fled Britain via Amsterdam in the company of Junaid Hussain, a Birmingham-based member of a computer hacking group who was on police bail for alleged violence.

Neither man is believed to have been ordered to surrender his passport despite their known views in support of jihad and the imposition of Sharia law.

When Aziz reached Syria he said he was undecided about what to do with his British passport, tweeting “could burn it [or] flush it down the toilet”.

Home Secretary Theresa May has been forced to defend the Government’s record on controlling terror suspects following the publication of the Isis execution video, acknowledging that more than 800 UK nationals of concern to the intelligence services have travelled to Syria.

Under new powers introduced in February, police can now temporarily seize the passports of individuals suspected of travelling to engage in terrorism abroad. Ms May said the powers had been used more than 20 times and in some cases led to the permanent removal of passports.

Home Secretary Theresa May making a statement to MPs on the latest Isis propaganda video, earlier this week (PA)

But problems remain with ensuring that individuals, such as Dhar and at least two other of his former Al Muhajiroun colleagues, who are ordered to surrender their passports as part of criminal investigations do so.

It emerged this week that Dhar fled Britain within 48 hours of being released on bail following his arrest on suspicion of encouraging terrorism but police were still unaware of his whereabouts at least a month afterwards, when they sent him a letter asking him to report to his nearest police station.

David Anderson QC, the Government’s independent reviewer of anti-terror legislation said that while it was impossible for police to monitor every individual, it was concerning that Dhar had been able to travel while under suspicion of a crime.

He told BBC News: “ What is unusual about this case is that he managed to make that journey despite being subject to the criminal justice process, despite having been arrested and placed under police bail.”