Boris Johnson was facing growing criticism on Saturday night for failing to cut short his Caribbean holiday as the Middle East faced one of the gravest crises since the Iraq war in 2003.

I have no confidence Boris Johnson will keep us out of a quagmire in Iran Read more

As global leaders weighed in on the assassination of Iran’s top general, Qassem Suleimani, in a US drone strike in Baghdad in the early hours of Friday, Johnson remained silent on the island of Mustique where he is on holiday with his girlfriend, Carrie Symonds.

Downing Street said Johnson would be back in the UK on Sunday and that the foreign secretary, Dominic Raab, would travel to Washington this week for talks with the US secretary of state, Mike Pompeo, with the Middle East the main topic of discussion. In a sign of irritation that the UK had not already expressed its support for the US, Pompeo on Friday said the Europeans, including “the Brits”, “[had]n’t been as helpful as I wish that they could be”.

On Saturday night the defence minister, Ben Wallace, announced that the warships HMS Montrose and HMS Defender would return to escort duties for ships sailing under a British merchant flag through the Strait of Hormuz. “The government will take all necessary steps to protect our ships and citizens at this time,” he said.

Johnson is expected to update MPs on the crisis when parliament returns after the Christmas and New Year break on Tuesday. But as the hashtag #wheresBoris began trending on Twitter, opposition politicians tore into Johnson, suggesting he was too afraid of upsetting President Donald Trump, who ordered the assassination of Suleimani, to speak out. Tory sources said his silence was more likely to be part of a co-ordinated effort by EU leaders to say little in order not to inflame the situation.

Play Video 1:37 Aftermath of US airstrike that killed top Iranian general Qassem Suleimani – video report

Shadow foreign secretary Emily Thornberry said it was “astonishing” that Johnson had still to make a statement on the crisis in Iran, which was “on the brink of becoming an even more destructive and catastrophic war than Iraq or Syria”.

Writing in the Observer, she says: “Is he afraid of angering President Trump? Or is it simply that, as he lounges in the Caribbean sun, he simply does not care, an exact mirror of the blase approach on Iran he took in his past role as foreign secretary when Trump was driving the nuclear deal to destruction in 2018, and when recklessly jeopardising the fate of Nazanin Zaghari-Ratcliffe the previous year.”

Ed Davey, the Liberal Democrats’ acting leader, said Johnson’s silence was “deafening”. “The prime minister must speak out now and make clear Britain will not support the United States in repeating the mistakes of the Iraq war,” he said.

Ian Blackford, the SNP’s leader at Westminster, said he found Johnson’s silence “quite extraordinary” adding: “There is a clear responsibility on him as prime minister of the United Kingdom to speak up in what is a very, very dangerous situation. All of us have got to call for calmness.”

Asked if he believed Johnson might be worried about upsetting his friend Trump, Blackford reiterated: “He is the prime minister of the United Kingdom and he should speak without fear or favour at such a time.”

A former Tory minister with experience of the EU and the Middle East said he believed Johnson’s decision not to speak out was part of a deliberate strategy, co-ordinated with other EU leaders. “We are still members of the EU [until 31 January] and I think it is right. Don’t get sucked into the mangle. Pause for thought. That is the correct response.”

As the Foreign Office on Saturday updated its advice to British travellers, warning against visiting Iraq apart from essential travel to the Kurdistan region, former foreign secretary Jeremy Hunt said the US was playing a risky game. “It’s an incredibly dangerous game of chicken going on because both sides have calculated that the other side cannot afford and doesn’t want to go to war,” he said.

“So they are doing increasingly extreme things, not just the assassination of Suleimani … the bombing of the Saudi oil facility last September is another example. It is true neither side wants to go to war but it’s also true that both sides are compelled to react when things like this happen. That is the risk in this situation.”