The Government is facing growing criticism for failing to condemn a US air strike in Iraq that killed a top Iranian general and heightened tensions in the Middle East.

On holiday in the Caribbean, Prime Minister Boris Johnson remained silent on Saturday over the drone strike on General Qassem Soleimani which threatened to provoke all-out war as Iran vowed harsh retaliation.

Foreign Secretary Dominic Raab has called for calm and urged all aggressors to de-escalate following Friday’s attack on the head of Iran’s elite Quds Force and mastermind of regional security strategy.

The Foreign Office issued strengthened travel advice to Britons across the Middle East, but Mr Raab pointedly did not criticise the strike.

The PM was urged to recall Parliament early and Labour’s John McDonnell said it is “not good enough” that the Government has not condemned US President Donald Trump for authorising the killing.

At an anti-war protest at Downing Street, the shadow chancellor vowed to press Mr Johnson over the attack, which he said will “set the Middle East and the globe alight yet again”.

Mr McDonnell told the Stop the War Coalition event: “We’ve been here before, we were here 17 years ago. And there’s one lesson that came from those events, is that violence begets violence.

More than 150 anti-war activists gathered outside Downing Street
More than 150 anti-war activists gathered for a protest outside Downing Street (Yui Mok/PA)

“And it’s not good enough for the UK Government just to appeal for a de-escalation, what we expect the UK Government to do is to come out in total and outright condemnation of this act of violence.

“We will not tolerate us being dragged yet again into this type of aggressive military action which puts us all at risk.”

The PM has been celebrating the new year with his partner Carrie Symonds on the private island of Mustique but is expected to return to the UK early on Sunday.

Outgoing Labour leader Jeremy Corbyn has requested an urgent meeting of the privy council, which advises the monarch, over the “assassination”.

Labour’s shadow business secretary Rebecca Long-Bailey, who is expected to enter the Labour leadership race, said: “We stand at a dangerous moment. Boris Johnson must urgently make a statement on what the UK Government is doing to avoid war.”

The protest was organised by the Stop the War Coalition
The protest was organised by the Stop the War Coalition (Yui Mok/PA)

Labour MP Lisa Nandy, as she launched her campaign to succeed Mr Corbyn, demanded the PM recalls Parliament to explain how he plans to “keep British personnel safe” in the region where hundreds of UK troops are based.

She said Mr Johnson must outline how he will work with European allies to ensure “a much more international and concerted action to rein in what has been quite a dangerous and reckless act by a president without real thought about what comes next”.

Shadow foreign secretary Emily Thornberry earlier said the Foreign Office’s call for restraint was “too little and far too late, in the wake of such a brazen, unlawful and provocative attack”.

More than 150 anti-war activists, some armed with placards, joined the Stop the War Coalition protest.

Shadow justice secretary Richard Burgon, who has made a bid to be Labour’s deputy leader, told the crowd the UK must not be “dragged” into any war with Iran.

He told the PA news agency he believes the US president pursued the attack for his own “electoral purposes”.

He said: “I and others marched against the Iraq war, shamefully our Government supported George Bush’s war in Iraq.

“It made life worse, even worse for people in Iraq, it made terrorism flourish, and it didn’t help people in the Middle East or around the world.

“Because Donald Trump, I think, is doing this for electoral purposes. I think it could be as cynical as that.

“And so we’ve got to argue against war, argue for peace, argue for conflict resolution, and argue in our ever more dangerous world that what we really need is to avoid the rush to war.”