Boris Johnson told ITV News that the Prime Minister would be right to hold a second vote on military action on Syria if there was "absolute proof that the Assad regime" had been involved in chemical attacks and there was "a limited and a calibrated response" put forward.
The Mayor of London added: "It is very, very hard to stand aside when you have people being killed in large numbers by chemical weapons".
Cameron and Miliband avoided any real debate on Syria today in Parliament, as they are both in odd and difficult positions.
For the time being it seems likely we are going to sit this one out and I think you can expect the debate on what that means to run and run.
Number 10 has stated it has "absolutely no plans" to go back to Parliament on the issue of military action against Syria.