In the latest indication that Conservative opinion is shifting sharply away from the idea of transferring arms to the rebels fighting Bashar al-Assad’s regime in Syria, Mr Johnson said that Britain “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.”
Mr Johnson’s comments, which came in his regular column in the Daily Telegraph, were another sign that Mr Cameron would find it hard to win a free vote in the Commons if he decided to dispatch arms to the rebels.
The prime minister has repeatedly said in recent days that no decision has been taken and that a vote would be offered to MPs before any arms were sent.
Last week, the Obama administration said it would arm the opposition. But since then, a range of prominent voices – including General Richard Dannatt, the former head of the British Army – have come out against a similar move by the UK.
Mr Cameron received some support on Monday from Sir Malcolm Rifkind, the former foreign secretary and chairman of the Commons Intelligence and Security Committee. Sir Malcolm said there was no reason why arms sent to moderate opposition forces would end up with jihadi elements who were their “sworn enemies”.
However, Ed Miliband, the Labour leader, has expressed concern about such a move and Liberal Democrat MPs say their own party is split on the issue. As a result, Mr Cameron would require a significant change of sentiment if he were to press ahead with such a move.
William Hague, foreign secretary, warned on Monday that without such weapons Syria’s opposition groups faced possible defeat. Mr Hague described the conflict in Syria as the “the worst human tragedy of our times and on a trajectory to get worse”.
He insisted that arms need not fall into the wrong hands and stressed that the UK was only sending non-lethal equipment to “moderate, more sensible elements of the opposition”.
Mr Hague said: “Obviously, we’re not sending it to extremist groups, to groups that we’re very concerned about, that could become a terrorist threat”.
He added: “The equipment we have supplied so far is not arms, but we have no evidence that it has fallen into the wrong hands in any sense and we have been supplying it for some time, so bear that in mind.”
Richard Ottaway, chairman of the foreign affairs select committee and a senior Tory MP, told the Financial Times on Monday that the leadership needed to think carefully about the outcomes of further intervention in Syria’s civil war. “If we do anything – and it is a big if – it should be to make a big difference, and bunging in a few cases of rifles won’t make much difference.”