David Cameron and William Hague argued today that Britain had a duty to “engage” in the Syria crisis despite mounting domestic criticism of their stance and fears that plans to arm rebel opposition forces could backfire.

The Prime Minister and Foreign Secretary warned that standing aside could make the crisis even worse because al-Qa’ida could dominate the rebels opposing the Assad regime. But the rising tensions in the Conservative Party over Mr Cameron’s plans to intervene were shown when Boris Johnson, the Mayor of London, declared that Britain could not end the conflict by “pressing weapons into the hands of maniacs.”

Writing in the Daily Telegraph, Mr Johnson said the UK would have no way of preventing weapons reaching “al-Qa’ida-affliated thugs” if it supplied the Syrian opposition following the EU’s decision to lift its arms embargo. “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.”

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Syria will dominate a two-day meeting of G8 leaders, chaired by Mr Cameron, starting today at Lough Erne near Enniskillen, Northern Ireland. The eight presidents and prime ministers will discuss the crisis over a working dinner tonight but British officials are not optimistic about a breakthrough. One-to-one talks are expected between Barack Obama and Vladimir Putin as they line up on both sides of the civil war following the United States’s decision to provide direct military support to the rebels.

Hitting back at the London Mayor, Mr Hague said Britain must prevent the Syrian opposition from being “exterminated”. The Foreign Secretary told BBC Radio 4: “Our fear is we are only going to get a political solution to this crisis if the opposition, the moderate sensible parts of the opposition, can't be destroyed. Therefore, they do need assistance of various kinds.“

Mr Hague conceded: “There are no easy options at all. Of course it is not easy to take any decision to send arms into a conflict. It's also not easy to take a decsion to allow people to be killed who are faced by much superior arms, and who may be driven to radicalism and extremism by being placed in that situation.

“It is the worst human tragedy of our times. It is on a trajectory to get worse. I don't want to understate in any way the severity and bleakness of this crisis.”

In a round of media interviews ahead of the G8 summit, Mr Cameron argued: “My argument is that we shouldn't accept that the only alternative to Assad is terrorism and violence. My concern is that the current trajectory in Syria is very, very damaging. You have got a dictator who is brutalising his people, who's using chemical weapons against innocent people. You've got an opposition, elements of which are extremely dangerous extremist terrorists.

”The trajectory we are on is very damaging, so what we need to do is make sure we can have a peace conference and a peace process and a transition to a regime that can stand up for everyone in Syria.

“It's an extraordinarily difficult situation, but I think we should be engaged rather than disengaged.”