Britain will have no means of preventing UK weapons ending up in the hands of "al Qaida-affliated thugs" if it supplies arms to the Syrian rebels, Boris Johnson has warned.
The London mayor put himself at the head of the growing opposition at Westminster to any move by David Cameron to arm the rebels, saying Britain could not end the conflict by "pressing weapons into the hands of maniacs".
The intervention by his old rival is a further headache for the Prime Minister as he prepares to host the G8 summit at Lough Erne in Northern Ireland where Syria will be high on the agenda.
In a round of TV interviews at Lough Erne, Cameron said he was "as worried as anybody else about elements of the Syrian opposition, who are extremists, who support terrorism and who are a great danger to our world.
"The question is what do we do about it? My argument is that we shouldn't accept that the only alternative to Assad is terrorism and violence."
Cameron has voiced his strong support for President Barack Obama who has said the US is to start arming the rebels after concluding that President Bashar al Assad has used chemical weapons against his own people.
However the scale of the opposition among MPs of all parties means Cameron - who has promised a Commons vote on any move by Britain to arm the rebels - will almost certainly find his path blocked if he tries to follow suit.
And Russia's Vladimir Putin issued a blunt warning on Sunday after a meeting with Cameron not to arm Syrian rebels who "eat the organs" of their enemies.
Writing in The Daily Telegraph, Johnson described an incident in Aleppo where a 15-year-old boy was taken away and beaten and then summarily executed by Islamist rebels for making a joking reference to the Prophet Mohammed.
"Odious, twisted, hate-filled thugs; arrogant and inadequate creeps, intoxicated by the pathetic illusion of power that comes with guns; poisoned by a perversion of religion into a contempt for all norms of civilised behaviour," he wrote.
"They are fighting not for freedom but for a terrifying Islamic state in which they would have the whip hand - and yet there is no dodging or fudging the matter: these are among the Syrian rebels who are hoping now to benefit from the flow of Western arms .
"How is it supposed to work? How are we meant to furnish machine guns and anti-tank weapons to one set of opposition forces, without them ending up in the hands of men like the al Qaida-affiliated thugs who executed a child for telling a joke?
"This is not the moment to send more arms. This is the moment for a total ceasefire, an end to the madness. We can't use Syria as an arena for geopolitical point-scoring or muscle-flexing, and we won't get a ceasefire by pressing weapons into the hands of maniacs."
Johnson's comments echoed former Liberal Democrat leader Lord Ashdown who said arming the rebels would be "an act of very considerable folly" that risked drawing Britain into a wider sectarian struggle between Sunni and Shia Muslims.
Tory backbencher Julian Lewis said it could be "suicidal" leading to extremist elements among the rebels gaining control of the regime's arsenal of chemical weapons.
The Archbishop of York, Dr John Sentamu, has also pitched in describing the idea of arming the rebels as "very, very naive" saying it would be impossible to know if the weapons were going to the "good guys" among the various factions.
"There are all kinds of very different factions and it may simply be a battle between Shiites and Sunnis, but also a battle within among themselves - the radicals, the conservatives, the reformists. It's a toxic mixture," he told Radio 5 Live's Pienaar's Politics.
Cameron told reporters on Monday: ""We should be on the side of Syrians who want a democratic and peaceful future for their country and one without the man who is currently using chemical weapons against them.
"What we can try and do here at the G8 is have further pressure for the peace conference and the transition that is needed to bring this conflict to an end."
The Prime Minister added: "We haven't made a decision to give any arms to the Syrian opposition but what we do need to do is bring about this peace conference and this transition, so that people in Syria can have a government that represents them, rather than a government that's trying to butcher them.
"What we are doing right now is helping the official Syrian opposition - people who have signed up to democracy and human rights, who want that sort of future for Syria.
"We are advising them, helping them and we are assisting them - and we should.
"President Assad wants us to think that the only alternative to him is extremism and violence. Yet there are millions of people in Syria who want a peaceful and democratic future. We should be on their side."