There may be as many as 2,000 Brits currently fighting with ISIS, a Muslim Labour MP has claimed. His estimate, based on his experience as a Member of Parliament for the predominantly Muslim constituency of Birmingham Perry Barr, is roughly four times as high as the official estimate released by the authorities before the summer.

His comments come amid growing fears that the British borders are porous to terrorists leaving for, and returning from fighting in Syria and Iraq, fuelled by the Home Office’s refusal to reveal how many people have been arrested at British ports and airports in relation to terrorist offences in Syria, the Sunday Telegraph has reported.

Khalid Mahmood, Britain’s first Muslim MP and a former member of the Home Office select committee told that paper “The authorities say there are 500 British jihadists but the likely figure is at least three to four times that. I think 2,000 is a better estimate. My experience in Birmingham is it is a huge, huge problem.

“The Government does not have significant people at border control. The fact is these jihadists are coming in and going out without almost ever being arrested. We have had hardly any arrests. We have had people coming back in after six months in Syria and they are not being picked up.”

At least four Muslims are known to have travelled to Syria in recent months despite having had their passports confiscated by the authorities or concerned parents, but the likelihood is that many more have done the same. Meanwhile, around 250 British jihadis are known to have returned to the UK from Syria and Iraq, yet only 30 arrests have been made, leaving a number of hardened fighters walking the streets of Britain.

Over the weekend it emerged that two more men, Abu Abdullah al Habashi, 21, and Abu Dharda, 20, both London natives, had died fighting in Syria. It is thought they were both killed in Kobane, the Syrian border town being defended by Kurdish forces. Dharda is known to have been questioned by anti-terrorism police attempting to board a flight to Turkey, but was allowed to travel because they were “satisfied with the explanation” he gave for his travel plans.

It is understood that Al Habashi, a British-Eritrean, spoke about his support for ISIS online, and appeared in two propaganda videos released by the group. Security services believe that around 35 Britons have been killed fighting in the region so far, but evidence from social media suggests their numbers are regularly replenished.

A British jihadist going by the name Qa’qa al-Biritani took to Twitter on November 11th to say “Was sat with around 20 British brothers. Allah has brought more to replace those who were martyred.”

The Muslim Council of Britain accused the government of not doing enough to prevent potential terrorists from leaving the country. “There are serious questions for the Home Office and the security services to answer. I don’t understand why border authorities are not making arrests,” said Harun Khan, deputy director general of the Council.

But the government insists that the border agencies are working hard to combat the threat. James Brokenshire, the immigration and security minister, said: “The intelligence agencies and police are working to identify and disrupt potential threats. This includes interviewing individuals at the UK border suspected of being involved in the commission, preparation or instigation of acts of terrorism, and cancelling or withdrawing the passports of some UK nationals of concern seeking to travel to Syria or Iraq.”

Home Secretary Theresa May will be unveiling her new Counter-Terrorism and Security Bill this week, designed to force internet companies to do their bit by holding on to data from internet users. She claims that the information will help counter-terrorism forces identify potential terrorists and uncover plots, but critics have labelled the bill a “snoopers charter”, arguing that the new powers are so wide ranging that they will be used to spy on citizens not suspected of terrorism offenses.

Emma Carr, director of Big Brother Watch said “Before setting her sights on reviving the snoopers charter, the Home Secretary should address the fact that one of the biggest challenges facing the police is making use of the huge volume of data that is already available, including data from social media and internet companies. The snoopers charter would not have addressed this, while diverting billions from investing in skills and training for the police.”