David Cameron is facing pressure to recall Parliament amid signs that the US and Britain are preparing for military action against Syria.
Labour and Tory backbenchers have insisted the Prime Minister should explain himself to MPs before intervening in the wake of alleged chemical weapon use by Bashar Assad's regime. Downing Street said summoning the Commons from its summer break had not been ruled out - but stressed Mr Cameron "reserved the ability to take action very swiftly if needed".
Earlier the Syria government attempted to take the edge off international criticism by agreeing to let UN inspectors visit the scene where chemical weapons were apparently deployed on Wednesday. A team of experts is expected to begin an investigation on Monday.
But Washington has accused the regime of having "something to hide" and delaying access for four days so that evidence degrades. An intense round of diplomacy on the crisis has also been continuing, with Mr Cameron and French president Francois Hollande warning that the "crime must not be swept under the carpet".
A Number 10 spokesman said the Prime Minister had also spoken to German chancellor Angela Merkel. "They agreed that this was a very grave incident and that there was little doubt that it had been carried out by the regime, particularly given their refusal to grant the UN access to the site immediately after the attack," the spokesman said.
"They agreed that UN investigators should visit the site tomorrow to seek to establish further information but we should be realistic about what they will be able to achieve and that, following continued shelling in recent days, any hard evidence is likely to have been corrupted. They agreed that such an attack demanded a firm response from the international community and to keep in touch in the days ahead."
On Saturday night Mr Cameron and Barack Obama spent 40 minutes discussing the situation, ordering officials to examine "all options". Mr Obama previously suggested that the use of chemical weapons in Syria would be a "game changer", but he has stopped short of committing to military intervention in the troubled country - a step that would risk a confrontation with Russia. Mr Cameron would also face strong domestic resistance, with scores of Conservative MPs having already argued there should be a Commons vote before arms are supplied to rebels.
Tory backbencher Andrew Bridgen said the premier should recall Parliament if there was a prospect of action. "I want to hear what the Prime Minister or the Foreign Secretary have got to say at the Despatch Box," he told BBC Radio 4's Westminster Hour. If there's a compelling case for us to be further involved in this - and it can't be unilaterally, it has to be as part of a coalition - then I'm there to be persuaded along with my colleagues. However, there is no good solution to this crisis and I would hope that if we had a parliamentary debate we could perhaps get to the least worst solution for the Syrian people and the UK."
Shadow foreign secretary Douglas Alexander said: "In light of ongoing international developments, it is right that the UK works with the international community to try and find an agreed way forward in Syria. If, in reality, the Prime Minister is now considering military options involving UK personnel then of course I would expect him to seek a recall of Parliament and to come to the House of Commons and make his case in advance of a decision being made."
A Downing Street spokesman stressed no decisions had been taken and there was no timetable for action. "The Prime Minister has made clear that MPs should have the opportunity to debate issues like this and that still stands although we must reserve the ability to take action very swiftly if needed," the spokesman said. "No decisions have been taken on military action and the timetable for a serious response from the international community is not yet clear. We are very conscious of when MPs are due to return and haven't ruled out recalling them earlier."