Britain is planning to join forces with America and launch military action against Syria within days in response to the gas attack believed to have been carried out by President Bashar al-Assad’s forces against his own people.
Royal Navy vessels are being readied to take part in a possible series of cruise missile strikes, alongside the United States, as military commanders finalise a list of potential targets.
Government sources said talks between the Prime Minister and international leaders, including Barack Obama, would continue, but that any military action that was agreed could begin within the next week.
As the preparations gathered pace, William Hague, the Foreign Secretary, warned that the world could not stand by and allow the Assad regime to use chemical weapons against the Syrian people “with impunity”.
Britain, the US and their allies must show Mr Assad that to perpetrate such an atrocity “is to cross a line and that the world will respond when that line is crossed”, he said.
British forces now look likely to be drawn into an intervention in the Syrian crisis after months of deliberation and international disagreement over how to respond to the bloody two-year civil war.
The possibility of such intervention will provoke demands for Parliament to be recalled this week.
The escalation comes as a direct response to what the Government is convinced was a gas attack perpetrated by Syrian forces on a civilian district of Damascus last Wednesday.
The Assad regime has been under mounting pressure to allow United Nations inspectors on to the site to establish who was to blame for the atrocity. One international agency said it had counted at least 355 people dead and 3,600 injured following the attack, while reports suggested the true death toll could be as high as 1,300.
Syrian state media accused rebel forces of using chemical agents, saying some government soldiers had suffocated as a result during fighting.
After days of delay, the Syrian government finally offered yesterday to allow a team of UN inspectors access to the area. However, Mr Hague suggested that this offer of access four days after the attack had come too late.
“We cannot in the 21st century allow the idea that chemical weapons can be used with impunity, that people can be killed in this way and that there are no consequences for it,” he said.
The Foreign Secretary said all the evidence “points in one direction”, to the use of illegal chemical agents by Assad regime forces.
A Government source added that even if UN inspectors visited the site of the attack, “we would need convincing by the UN team that this was not the regime’s attack because we believe everything points to the fact that it was”.
Officials said the Assad regime has continued bombarding the area in the days since the attack, making it likely that any evidence which could establish who was responsible will have been destroyed.
Mr Cameron interrupted his holiday in Cornwall for talks with Mr Obama, François Hollande, the French president, and Angela Merkel, the German chancellor. After discussions via a secure telephone line over the weekend, all the leaders agreed on the need for a “serious response”. Government sources confirmed that military action was among the options “on the table” but said no decisions had been taken.
The Prime Minister, however, is believed to have abandoned hope of securing any further meaningful response from the UN amid opposition from Russia.
Labour said Parliament must be recalled if Mr Cameron was considering a military response, but Downing Street sources said this may not be necessary as the Prime Minister retained the right to act urgently if required.
Mr Cameron will face criticism for any British military involvement from many MPs, who believe the Armed Forces are already overstretched and must not be committed to another distant conflict.
Any retaliatory attack would be likely to be launched from the sea as the Syrian air force is judged to be strong enough to shoot down enemy jets.
A Royal Navy nuclear-powered submarine is said to be in the region while a number of warships recently left Britain for exercises in the Mediterranean.
Commanders may also need to make use of the RAF base at Akrotiri, Cyprus for air support.
If military action is approved, the first wave of missiles could start within a week.
Military sources suggested the early hours of the 2011 campaign against Col Muammar Gaddafi could form a template for any operation. The Libya campaign began with a blitz of Tomahawk cruise missiles from US warships and from a British Trafalgar Class submarine.
The Royal Navy declined to comment on the current positions of its submarines, but they regularly pass through the area on their way to the Suez Canal.
America’s Sixth Fleet currently has four guided missile destroyers in the area, each of which could join the attack.
The Royal Navy also has its rapid response task force in the Mediterranean. The group includes two frigates and the helicopter carrier HMS Illustrious.
Navy sources said there were no plans to change the exercises, but the group provided “strategic contingency” if needed.