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A hardcore group of 50 British jihadis who have been fighting in Syria have returned to the UK and are planning to attack us, MI5 fears.

Intelligence officers believe the killers – experts in explosives and weapons – are out for revenge after the Home Office vowed to crack down on “terror tourists” battling and training abroad.

Around 250 jihadis are thought to have returned, but 50 are said to be plotting another attack like 7/7.

Homegrown fanatics are going to war-torn Syria to learn deadly skills but there have been recent threats of laws banning them from fighting abroad.

A security source said: “The very laws banning their jihad travels may goad so-called reformed jihadists into violence. Having had a taste of the frontline and seeing fellow extremists become martyrs they could turn on mainland Britain.

“It is very important extremist groups are identified and monitored to make sure they do not endanger British lives.”

Anti-terror police have stepped up the monitoring of extremist mosques and border officials are tightening checks on suspects trying to return from Syria.

Arrests linked to Syria are soaring, with 16 so far this year, compared to 24 for all of 2013. Extremists can get life in jail for “acts preparatory to terrorism and assisting a person in such activities” and they can get 10 years for “training as a terrorist”.

Abdul Waheed Majeed, of Crawley, West Sussex, blew himself up earlier this month by driving a Mad Max-style lorry full of explosives through the walls of Aleppo prison in Syria.

In the UK Majeed, 41, had lived in Martyrs Avenue. He left England six months ago to join a convoy running aid into Syria. Extremists in Syria have formed a “British Battalion,” known for torture and killing.

Shiraz Maher, of the International Centre for the Study of Radicalisation at King’s College London, said: “The Brits in Syria fight hard. Even though they do not plan to come back, some accept it is possible they will have to one day.

“Regarding the risk to Britain, bear in mind that post 9/11 most attempted attacks were bungled as the terrorists didn’t know what they were doing.

“But Syria is a permissive arena for blowing things up and shooting at people so they know what they are doing with guns and explosives and it’s a genuine worry.”

MI5 is desperate to avoid a repeat of the 2005 7/7 terror suicide bombings, when four homegrown extremists blew up London Tubes and a bus, killing 52 and injuring more than 700.

Subsequent attacks on the country have largely failed. On July 21, 2005 – two weeks after 7/7 – four attempted bombings in London were thwarted when only the detonators went off.

Muktar Ibrahim, 29, Yassin Omar, 26, Ramzi Mohammed, 25, and Hussain Osman, 28, were found guilty of conspiracy to murder and jailed for life.

And in 2007 bombers tried unsuccessfully to smash an explosives-packed car into Glasgow Airport.

The driver Kafeel Ahmed, 27, died but no one else was seriously hurt. His companion Bilal Abdullah, 33, was found guilty of conspiracy to commit murder and sentenced to 32 years in prison.

Despite the sentences, the prospect of jail does not appear to have dampened extremists’ taste for terror – as proved by last year’s Woolwich attack on soldier Lee Rigby.

British Muslim converts Michael Adebolajo, 29, and Michael Adebowale, 22, ran Lee, 25, down in a car before hacking him to death with a cleaver and claiming they were “soldiers of Allah”.

It is thought there are about 400 fighters from the UK in Syria. Many have teamed up with bloodthirsty al-Qaeda-backed Jabhat al-Nusra and a small number are with Islamic State of Iraq and Sham, which al-Qaeda dropped.

Met Commissioner Sir Bernard Hogan-Howe said recently: “We are concerned about the number of British people going to Syria for humanitarian reasons or to get involved in the war.

“When they return and they are ­radicalised, they may be militarised, they may have a network of people that train them to use weapons.”

It is also thought women have been used to smuggle cash from Britain to terrorists in Syria, and in recent months it is claimed more than £50,000 has been seized from “money-mules” pretending the cash was for aid.

In January Londoner Nawal Msaad, 26, was accused of trying to smuggle £16,500 in her underwear at Heathrow.

She, and alleged co-conspirator Amal Elwahabi, 27, were the first British women charged with terror offences over the conflict. The pair are awaiting trial accused of being part of making money available for terrorism.

Uni lad who lived at home is one of 20 dead fanatics

An engineering student from a posh city suburb is the latest of around 20 British fighters killed on the Syrian front line.

Amateur boxer Anil Khalil Raoufi, 20, lived with his parents and was studying at university before he left for the war-torn country. He often boasted on Twitter about the glory of fighting – until he was shot in the head in battle.

He had lived in the affluent area of Didsbury, Manchester, and went to university in Liverpool.

His death followed that of dad-of- three Abdul Waheed Majeed, the first British suicide bomber to kill himself in Syria. Originally from Crawley, West Sussex, he had driven a lorry until heading out to Syria nine months ago.

Akram Sebah, 24, and his brother Mohamed, 28, were hailed as “Lions of Britain” by al-Qaeda chiefs, who used them as poster boys to lure more vulnerable Muslims before they were killed.

Two brothers from West London have also been killed fighting in Syria.

Among the most notorious British fighter was chef Ifthekar Jaman, 23, who died during an assault on a major arms depot with Isis, the most brutal al-Qaeda splinter group in Syria’s civil war.

He once appeared on BBC Newsnight and had become a jihadi celebrity for his online advice for fellow British extremists, including women he urged to marry men from the front lines.