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 warned.
Writing for the Daily Telegraph 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 Archbishop of York, Dr John Sentamu, who was making his first public appearance since he revealed he had prostate cancer, has described the idea of arming the Syrian rebels as "very, very naive."
"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 BBC Radio 5 Live's Pienaar's Politics.
Dr Sentamu also criticised corrupt government officials who embezzle funds from the public purse, at an ecumenical service in Enniskillen, Northern Ireland, ahead of the G8 summit.
"They are directly robbing the poor of their education, health, food, employment and sustainable development," he said.
"We say, from this fantastic cathedral to both types of robbers, 'enough is enough. Stop indirectly and directly robbing the poor'."
Russian President Vladimir Putin alluded to a graphic video, purporting to show a Syrian rebel commander biting into the heart of a soldier, during a joint press conference with the Prime Minister.
He made the point to justify Russia's opposition to arming Syrian rebels, saying that both sides in the conflict had "blood on their hands".
Putin also said he hoped the G8 Summit would have a "positive influence" on international efforts to end the conflict in Syria.
Russia and Britain can overcome their differences on the conflict in Syria despite some disagreements, Prime Minister David Cameron said during a joint press conference with President Putin:
What I take from our conversation today is that we can overcome these differences if we recognise that we share some fundamental aims: to end the conflict, to stop Syria breaking apart, to let the Syrian people decide who governs them and to take the fight to the extremists and defeat them.
The Prime Minister clarified that the UK has "made no decision to arm the rebels," but he said it was important to continue to train and help the opposition.
The Prime Minister has said that he believes Syrian President Bashar al-Assad "has to go" in order to bring peace to the country.
He described Assad as a "murderous dictator," adding: "I believe we must support the centre ground of decent moderate Syrians".
He said he thought that the differences between the UK and Russia could be overcome, and he hopes that the G8 talks will bring momentum to this process.
The Prime Minister has said he "does not see eye to eye" with Russia on all issues, but he said that the discussion with Russian President Vladimir Putin had been "serious and honest".
The comment came during a joint press conference at Downing Street.
The Russian President had to walk into Downing Street by the back steps because of a group of Turkish protesters outside the entrance. He seemed to take it in good humour.
Russian President Vladimir Putin has arrived in Downing Street for talks with the Prime Minister about the conflict in Syria.