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.
Mr 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 Mr 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.
Writing in The Daily Telegraph, Mr 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."
His comment 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.
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