Britain’s Armed Forces are drawing up options for a joint strike with the US on Syrian forces.

Military chiefs were yesterday instructed by ministers to plan for expanded operations against Bashar Assad in the wake of a poison gas atrocity blamed on the dictator.

No 10 refused to rule out the Prime Minister recalling Parliament to vote on action but sources said she might be able to go ahead without approval from MPs.

How Britain could wound Assad: The different weapons and methods which could be employed against the Syrian dictator

Talks are underway across Whitehall on the possibility of bringing back MPs from the Easter recess to debate a response, with insiders saying it was a ‘grey area’ as to whether they were needed.

David Cameron lost a Commons vote in 2013 over launching airstrikes on the Assad regime following a chemical weapons outrage.

There is no legal requirement on the Government to seek parliamentary approval before military action but Mr Cameron set a precedent on consulting MPs first.

The talks came as Theresa May said President Assad’s main backer, Russia, must be held to account if the regime is found to be responsible for the chemical attack near Damascus.

Describing the attack on Saturday which killed dozens of civilians, including children, as barbaric, she refused to rule out a military response.

‘We are working urgently with our allies to assess what has happened,’ she said. ‘We are also working with our allies on any action that is necessary.’

Senior MPs have urged the PM to strike. Former aid minister Priti Patel tweeted yesterday: ‘Syria has become a proxy battlefield with another act of barbarism by Assad sanctioned by his Russian and Iranian backers.

‘World leaders can no longer say that strong action is needed unless they are prepared to act. Inaction has become a stain on our collective humanity.’

Syrian dictator Bashar Assad, who has been blamed for the chemical attack near Damascus

The RAF is operating over Syria in missions exclusively against Islamic State targets.

But eight GR4 Tornado jets based at RAF Akrotiri in Cyprus could fire precision-guided Storm Shadow missiles at Syrian military bases.

A Royal Navy submarine could also fire Tomahawk cruise missiles.

Donald Trump said he would be seeking opinion from his military advisers about possible responses, adding ‘nothing’s off the table’.

Last night an American destroyer, the USS Donald Cook, was positioning itself in the eastern Mediterranean.

It also has Tomahawks, the weapon used by Washington a year ago in a punitive strike on the Syrian regime following a gas attack on civilians.

Theresa May (pictured) said President Assad’s main backer, Russia, must be held to account if the regime is found to be responsible for the chemical attack

Foreign Secretary Boris Johnson has told his French counterpart, Jean-Yves Le Drian, that ‘a full range of options should be on the table’.

Sources said any UK response would be taken along with the US, France and others.

‘This is not going to happen imminently,’ said a Whitehall insider.

‘It is early days but we have the assets in play. They are ready in our back pocket to use if necessary but this is still in its early stages.’

Asked yesterday whether Russia’s Vladimir Putin bears responsibility for the atrocity on Saturday in Douma, Mr Trump said: ‘He may, yeah, he may. And if he does it’s going to be very tough, very tough. Everybody’s gonna pay a price. He will, everybody will.’

Volunteers give aid to children at a hospital following the chemical attack

He said the US was investigating the possible involvement of the Iranian and Russian governments, adding: ‘Nothing’s off the table. It was an atrocious attack.’

The President’s defence secretary Jim Mattis is consulting with allies in Europe, the Middle East and elsewhere.

At a press conference during a visit to Denmark, Mrs May said the attack in Douma – near the capital Damascus – was part of a ‘troubling wider pattern of acts of aggression’.

Russian ambassador to the UN Vassily Nebenzia told an emergency meeting of the Security Council yesterday that his country was being threatened by the US and UK.

He said: ‘The tone with which this is being done has gone beyond the threshold of what is acceptable, even during the Cold War.’

The attack came amid a resumed offensive by Syrian government forces after the collapse of a truce with a rebel group.

Opposition activists and rescuers said poison gas was used – an allegation strongly denied by the Assad regime.