David Cameron will respond to mounting demands by MPs to recall Parliament to discuss Syria as Britain and international allies contemplate launching military action over the use of chemical weapons.
The Prime Minister has returned 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 against the Assad regime.
America significantly toughened its rhetoric over what it described as the "undeniable" use of banned nerve agents by the Syrian government against an opposition-held suburb of the capital Damascus. US secretary of state John Kerry said the attack, which doctors say killed hundreds, was a "moral obscenity" that "should shock the conscience of the world" and promised action to hold the regime accountable.
The mood darkened further when United Nations weapons inspectors investigating the claims came under sniper fire as they drove to the area despite assurances of their safety from both sides in the civil war.
As he continued a round of diplomatic calls with world leaders, Mr Cameron clashed with Vladimir Putin over Russia's continued insistence that there is "no evidence" of a chemical attack.
Assad denies using the weapons and Moscow - a key regime ally which supplies arms to Syria - has backed claims video footage of victims could be opposition propaganda. It says military action would be a violation of international law and doomed to fail.
The Prime Minister told Mr Putin there was "little doubt" the regime had used the weapons and then acted to cover up the evidence for five days before allowing the inspectors in, showing it had "something to hide".
Foreign Secretary William Hague suggested force could be legal even if Russia vetoed UN Security Council backing and declined to rule out action, such as targeted air strikes, being launched within days. Any intervention would be "in accordance with international law and will be based on legal advice to the national security council and to the Cabinet", Mr Hague stressed.
Deputy Prime Minister Nick Clegg has cancelled a visit to UK troops serving in Afghanistan so that he can attend the NSC meeting. A spokesman for Mr Clegg said he supported the need for a "strong response" from the international community to the "abhorrent" use of chemical weapons.
The party leaders face significant opposition to British involvement in military action - which may be exposed if Downing Street, as expected, decides to accept cross-party calls for Parliament to be recalled. A Downing Street spokesman said the Government would decide "whether the timetable for our response means it will be necessary to recall MPs sooner than Monday when the House is currently due to return".