MPs are to be recalled from their summer break to vote on whether Britain should take part in military action in response to the alleged use of chemical weapons by the Assad regime in Syria, David Cameron has said.

The prime minister said the Commons speaker, John Bercow, had agreed to his request to recall parliament from its summer recess on Thursday.

Cameron tweeted: "Speaker agrees my request to recall Parliament on Thurs. There'll be a clear Govt motion & vote on UK response to chemical weapons attacks."

The prime minister returned to Downing Street early from his summer holiday in Cornwall to allow him to chair a meeting on Wednesday of the national security council, which will consider evidence linking the Assad regime to the chemical attack last week.

Cameron is expected to make a statement to MPs on Thursday outlining what No 10 has described as compelling evidence of the regime's involvement in the attack.

The prime minister wants to build a strong case linking the regime to the attack in the east Ghouta region of Damascus to ensure there is a proper legal basis for any attack. Britain and its partners are unlikely to seek a UN security council resolution authorising military action because that would be vetoed by Russia.

Instead the allies will probably rely on international law banning the use of chemical weapons. Dominic Grieve, the attorney general, is likely to advise that this will be lawful only if there is definitive proof linking the Assad regime to the attack.

Downing Street is hoping to reduce the number of Tory rebels by acting in a deliberative manner. There were signs that this approach could be paying off when Julian Lewis, a strong opponent of widespread military action, said he could support a limited surgical strike.

The announcement by Cameron came after No 10 confirmed that Britain's armed forces were making contingency plans for a possible military strike against the Assad regime to deter the "abhorrent" use of chemical weapons. In a sign of the increased pace of activity, the prime minister's spokesman said plans for a military strike were under way.

The spokesman said: "I am not going to get into details on any specifics. All I would say is it is reasonable to assume that our armed forces are making contingency plans."

Downing Street said any military action would be designed to act as a deterrent against the future use of chemical weapons by the Assad regime and by others around the world. The prime minister's spokesman said any action would be fully in line with international law.

"Any use of chemical weapons is completely abhorrent and against all international law," the spokesman said. "In terms of endgame, 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."

Downing Street, along with the US and France, is waiting to see the findings of the UN inspectors who saw the sites of alleged chemical attacks in the east Ghouta region of Damascus on Monday. But the allies would not be bound by the UN findings.

The No 10 spokesman said: "We will see what the UN produces. But, as the foreign secretary has said, it is possible that given the regime prevented that UN team from going in on day one, the evidence from the site could well have been tampered with, moved or degraded."

The spokesman said this would be considered alongside the evidence that had been amassed by Britain, the US and France. Asked whether the response to the chemical attack would depend on the UN findings, the spokesman said: "Yes. What I am saying is there is a process that is going on. We are in discussions with our international partners looking at the evidence that is available."

The spokesman said of plans to recall parliament: "The prime minister's view has been consistent throughout, which is that it is important that parliament has an opportunity to debate and discuss these very important matters."

Michael Gove, the education secretary, suggested the decision on military strikes in Syria was one for Cameron and his advisers, rather than parliament.

"I think the decision about how the government is going to respond to the horrendous humanitarian atrocities we have witnessed is one which is properly taken by the prime minister and the members of the national security council," he said after a speech in central London on Tuesday.

"I know the prime minister has said in the past he respects the right of the House of Commons to be kept up to date. I don't think there has been any foreign secretary who has been as assiduous in keeping the House of Commons up to date with not just what's been happening in Syria but with what's been happening elsewhere in the Middle East as William Hague.

"So, I am absolutely confident the prime minister and the foreign secretary are the right people to be leading at this time and I think their response so far has been absolutely right in the face of what are horrendous crimes."