British jihadists make up the largest foreign contingent of one of the most violent terrorist groups in Syria, now infamous for beheading, crucifying and stoning to death enemies.

Syrian rebel commander Brigadier-General Abdulellah al-Basheer has urged the UK to send weapons to help fight Sunni Islamist group The Islamic State in Iraq and Syria (ISIS).

Almost two out of three of ISIS' fighters are foreign-born and have chosen to join a group bent on creating an Islamic state in the war-torn country and Iraq.

The group is so extreme that it has even been denounced by Al Qaeda.

Terrifying: A man believed to have been crucified by fighters from the Islamic State of Iraq and Syria (ISIS) in the northern Syrian city of Raqqa

Dangerous: Security forces battle with ISIS members, a group where two out of three fighters are foreign and the largest contingent are from Britain, it has emerged

Earlier this month the Foreign Office said it was investigating into reports that two British nationals died fighting for ISIS.

UP TO 700 BRITONS MAY BE FIGHTING IN SYRIAN CONFLICT

Hundreds of British jihadists have travelled to Syria to fight. Last week Mashudur Choudhury, 31, right, became the first person in the UK to be convicted of terrorist offences in connection with the Syria conflict. The UK’s police counter-terrorism chief Deputy Assistant Commissioner Helen Ball has admitted authorities are powerless to stop young Muslims heading to the war-torn state.

The Scotland Yard chief said the battle against the Assad regime acts like a ‘magnet’ – and fighters now number in the ‘mid’ rather than ‘low’ hundreds.

There is speculation the figure could be as high as 700 as the authorities admit they are struggling to monitor those who are in the Middle East. A national campaign has been launched to urge women to stop their loved ones going to Syria.

The mothers, wives and sisters of potential jihadists were asked to contact the police so that officers can intervene.

More than 40 people have been arrested for crimes linked to Syria in the first three months of this year, compared to 25 in the whole of 2013.+ In April prospective Brighton University student Abdullah Deghayes, 18, was killed in a gunfight in Kassab in Latakia. His two brothers, Jaffar, 16, and Amer, 20, remain in Syria.

Around 400 Britons are believed to have gone to Syria over the last two years to fight, with an estimated 20 having died.

Last week Mashudur Choudhury, 31, became the first person in the UK to be convicted of terrorist offences in connection with the Syria conflict after he went to the Middle Eastern country with the intention of joining a terrorist training camp last October.

In a letter to The Times, General al-Basheer, chief of staff of the supreme military council, the commanding body of the rebel Free Syrian Army (FSA), which opposes the regime of President Assad, said the 'majority' of ISIS fighters were from the UK, with others from France, Germany and Belgium.

He said: 'We, the Syrian people now experience beheadings, crucifixions, beatings, murders, outdated methods of treating women, an obsolete approach to governing society. Many who participate in these activities are British.'

The general also called for the British government to send weapons to help deal with ISIS, warning that a failure to act would increase the threat of terrorism in Britain.

He said: 'If ISIS is allowed to expand, these terrorists, having put their skills to the test in my country, will return to their homelands, perhaps to the UK, and continue on their pernicious path of destruction...

'We are at a critical juncture in our fight against violent extremism and hope that the UK and US can shrug off their fear of supporting us. A failure to actively support the FSA now will lead to ISIS' successes internationally.'

He added: 'The FSA can only go so far with the little we have. The UK and US governments must support us to defeat terrorism in Syria and prevent it from being exported to Europe and the US.'

Protests: Mourners chant slogans against the Al Qaeda breakaway group ISIS while carrying a flag-draped coffin of Ahmed Marzouk an Iraqi officer who was killed by them last week

Britain provides non-lethal and technical support such as body armour and communications to moderate Syrian rebels, The Times said, but has ruled out handing over weapons for fear of the falling into the hands of extremists.

Former Liberal Democrat leader Sir Menzies Campbell has called for a new debate on policy over Syria, saying: 'It's high time that we revisited the issue of Syria in the House of Commons.'