Mr Johnson said 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.
Is he merely trying to head off UKIP?