The Prime Minister dismissed suggestions that the UK was warming to the Syrian president, who is engaged in a fight against Isis and other militants.
“He can have no part in the future of Syria. I’m very clear: we haven’t even started discussions – we need to start discussions about how you bring a transition about,” he told BBC Radio 4’s Today programme on Tuesday.
“It’s not just my view that you can’t end up with Assad having a role in Syria: the Syrian people wouldn’t accept it.
“What you need to do is find a government that can appeal to Alawites, Kurds, Sunnis and Christians and if you don’t have someone who can do that you won’t have a Syria that works.”
The PM refused to say how long Mr Assad should stay in power as part of a transition however.
Foreign Secretary Philip Hammond hinted earlier this week that Mr Assad could stay in power for months to end the conflict in the region, in which hundreds of thousands have been killed and millions displaced.
"If the price for doing that is that we have to accept that Assad will remain as titular head of state for period of time, do I really care if that's three days, three weeks, three months or even longer? I don't think I do,” he said at Conservative party conference in Manchester.
In the last parliament the British government had wanted to launch bombing strikes against the Assad’s regime. The UK however now wants to bomb Isis, who the Assad regime is fighting.
The situation has been complicated in recent weeks after Russian military jets began intervention in Syria against militant groups, effectively on the side of the Assad regime.
Russia says it is bombing Isis, but US government sources say Syrian rebels trained by American intelligence agencies are also being targeted.
The Syrian civil war started in 2011 in the context of the Arab Spring protests but has since spun out of control into a fully-fledged armed conflict.
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