500 of the 2,000 estimated foreign ISIS fighters are believed to be BRITISH

At least one in four of the estimated 2,000 foreigners fighting for ISIS in Syria and Iraq is British - and half of those are already back in the UK, it has emerged today.

Serious questions have been raised as to whether enough is being done to stem the flow of fighters after the Government revealed it has only seized 23 passports this year to prevent them travelling to the war zone.

The Government says there are around 500 British among the fighters while a further 250 are thought to have already returned to the UK where the police and security services are attempting to watch them.

Experts also say they are also the most 'arrogant, vicious and vociferous' jihadists - and have been able to fly with ease fly to Istanbul in Turkey and jump on a bus to get into neighbouring Syria. 

Here are some of the most notorious Britons fighting for ISIS:

Abu Abdullah Al Brittani, 32, who used a social network to produce a travel guide for would-be jihadists

 

Primark supervisor Muhammad Rahman left his high street job for Syria, but died in a gunfight

Mashudur Choudhury, 31, became the first Briton to be found guilty of travelling to Syria to fight in the civil war. He failed the selection process to join extremisrs and blew cash borrowed from his family to get there on foreign holidays, a luxury car and prostitutes

Brothers Abdullah 18, Jaffar, 16, and Amer Deghayes, 20, pictured, are fighting in Syria. Amer described how his brother died with a 'laugh and a smile' because he believes he became a martyr

Reyaad Khan, appeared in a shocking recruitment video aimed at luring jihadists to Syria and could face a war crimes trial as he repeatedly uploaded bloodthirsty posts and pictures on social media

Former drug dealer Aine Davis left his wife for Syria. She was found guilty of trying to send cash to him. The gangmember also asked her if he could take more wives 

Abdul Waheed Majeed was a father-of-three from Sussex who became the first Briton to blow himself up

That means there are now almost three times as many British Muslims fighting for Islamic State than there are serving in the UK military, an MP has claimed.

But Khalid Mahmood, Labour MP for Birmingham Perry Barr, told Channel 4 News the lack of border controls meant the Government’s estimates of jihadi numbers were too low. ‘I think it’s about 2,000 people,’ he added.

According to the Ministry of Defence, there are around 600 British Muslims currently serving in the Armed Forces - just 0.4 per cent of total personnel. This is far below the 4.3 per cent of the British population as a whole who are Muslim. 

Irish police also believe that 30 jihadists are using the country as a base  while travelling to and from Syria and Iraq.

The murder of James Foley apparently by 'John' - one of a gang of British jihadis known as 'The Beatles' - has raised fears the killing fields of Syria and Iraq are dominated by extremist Britons.