Britain's armed forces are drawing up contingency plans for military action in response to the chemical weapons attack in Syria, Downing Street said.

David Cameron will continue talks with international leaders to agree a "proportionate response" that will "deter" Bashar Assad's regime from using toxic agents on the Syrian population, officials said.

No 10 indicated that a decision could be taken before the results of a report by UN weapons inspectors into the attack is produced but insisted any response would adhere to international law.

A decision on whether to recall Parliament this week will be made later on Tuesday, officials said.

Downing Street said all options were still on the table but insisted that no decision has been made over what form of action to take. A spokesman said: "No decision has yet been taken. We are continuing to discuss with our international partners what the right response should be, but, as part of this, we are making contingency plans for the armed forces."

The Prime Minister is returning early to Downing Street from a family holiday to prepare for a national security council (NSC) meeting on Wednesday which will discuss possible UK involvement in using force.

MPs are due back from their summer break next Monday but Labour and a growing number of backbench MPs from all parties - including many Tories - are demanding a chance to debate the situation more quickly.

No 10 refused to be drawn on whether any debate in Parliament would include a vote by MPs. "We will make an announcement later today about whether or not there will be an early recall to discuss these issues. The Prime Minister is back in Downing Street looking at it today, but I think that, throughout, any action that is taken on the basis of the discussions that we have with our international partners and allies, that any action that is taken that we make a clear case for why that action is being taken."

Mr Cameron told international leaders, including US president Barack Obama and Russian president Vladimir Putin, over recent days that any use of chemical weapons is "completely and utterly abhorrent and unacceptable" and the international community "needs to respond". A spokesman added: "In terms of end game, this is about looking at how we deter the use of chemical weapons because this is something that is completely abhorrent and against all international law. This is about deterring the use of chemical weapons."

The Government is looking at a "range of evidence" about the atrocity that international medical humanitarian organisation Medecins Sans Frontieres (Doctors Without Borders) (MSF) said killed 355 people. A spokesman said: "There is evidence that will come back from the UN so we will look at that but there is still a parallel process going on which is looking at the evidence we already have, the evidence that the Americans and our other international partners have." Asked if a decision on action was not necessarily dependent on the results from UN weapons inspectors, the No 10 spokesman said: "Yes. What I am saying here is there is a process which is going on and we are in discussions with our international partners looking at the evidence that is available."