British jihadists make up the largest foreign contingent of one of the most violent terrorist groups in Syria, a leading opposition commander has said.
Brigadier-General Abdulellah al-Basheer said more than 60% of foreign fighters in Syria belonged to The Islamic State in Iraq and the Levant (ISIL), a Sunni Islamist group blamed for beheadings and crucifixions as it attempts to create a religious-led Islamic state in the war-torn country and Iraq.
The group is so extreme that it has even been denounced by al Qaida.
Earlier this month the Foreign Office said it was investigating into reports that two British nationals died fighting for ISIL.
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 ISIL 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He said: "We, the Syrian people now experience beheadings, crucifixions, beatings, murders, outdated methods of treating women, an obsolete approach to governing society.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 ISIL, warning that a failure to act would increase the threat of terrorism in Britain.
He said: "If ISIL 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 ISIL's 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."
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 Hosue of Commons."
Around 400 Britons are believed to have gone to Syria over the last two years to fight, with an estimated 20 having died.