Thousands of illegal immigrants are disappearing into the UK every year, raising serious questions over how well Britain can defend itself against jihadists entering the country.

According to Home Office data obtained by the Sunday Telegraph, there has been a massive increase the proportion of immigrants who disappear after being stopped at border control for carrying false documents.

A total of 3,527 migrants absconded last year after failing border checks, of whom only 846 were later found, meaning that 76 percent escaped into Britain.

Although the total number of illegal immigrants who absconded decreased compared to the previous year – from 9,362 to 3,527 – security forces were getting worse at catching those who did abscond. In 2010, 58 percent of absconders were later caught, but only 24 percent were caught last year.

The true scale of the problem is likely to be even higher, however, as the figures only include immigrants who have actually been challenged by border guards.

Home Affairs Select Committee chairman, Keith Vaz, told the Telegraph: “These figures are of real concern and a worry in the context of the current security situation.

“It is of particular concern given the threat from returning British jihadists. We should be getting better, not worse, at dealing with those who are absconding. I will be seeking an urgent explanation from the Home Secretary as to why this is happening.”

David Hanson, a Labour Party shadow minister for home affairs, added that the figures show how the UK’s borders “simply weren’t protected”.

“Fewer people who abscond are now being tracked down and caught. Only 24 per cent are being picked up. This is totally unacceptable,” Hanson said.

The figures come as fears grow over the threat posed by British citizens fighting for ISIS. Earlier this week, the terror group released a video of U.S. journalist James Foley being beheaded by a masked jihadist with a British accent.

The UK government believes that around 500 British-born Muslims are fighting in Syria and Iraq, causing fears that battle-hardened jihadists could try to return and spread terror in the UK.