Boris Johnson has been accused of “sunning himself and drinking vodka martinis” instead of dealing with the crisis in the Middle.
Emily Thornberry, the shadow foreign secretary and a Labour leadership candidate, accused Mr Johnson of a lack of responsibility and said the government was doing "too little, too late".
Mr Johnson is on holiday with partner Carrie Symonds in the private Caribbean island of Mustique and is due to return to work on Monday after flying back on Sunday .
In a withering attack on Sunday, Ms Thornbery said there had been three emergency Cobra meeting Mark Sedwill, the cabinet secretary, had been forced to chair in Mr Johnson's absence.
Ms Thornberry told Sky News’ Sophy Ridge on Sunday programme: "We should take responsibility, we are international players, of course, we have other preoccupations, and clearly the Prime Minister has a lot of preoccupations - he's sunning himself drinking vodka martinis somewhere else and not paying attention to this.
"We've had three Cobra meetings where Mark Sedwill, the chief civil servant, has had to chair it because the Prime Minister hasn't been available."
Writing in the Observer, the Labour leadership contender suggested the prime minister was staying silent because he was “afraid of angering Trump”.
Repeating some of her claims, she accused Mr Johnson of previously dismissing her concerns that Mr Trump was heading down a dangerous path over Iran.
She told the programme: "I remember saying to Boris Johnson, 'I'm really worried that the president is going to rip up the Iranian nuclear deal,' and he said to me, 'You should spend a bit less time reading the newspapers'".
She said she voiced her "concerns about where the Americans were going and what was going to happen".
One reason she raised this, she said, was because of warnings from the chief of staff of the former US national security adviser Colin Powell that the "same path" was being followed as before the Iraq war.
But she said Mr Powell warned that war with Iran could be up to 15 times worse because of loss of life and other costs.
The attack came as Dominic Raab, the foreign secretary, said Mr Johnson would not be “back in play” until Monday, after returning to the UK this weekend .
But Mr Raab insisted he had been in “constant contact”, to discuss the crisis, adding: "What really matters here is that the Government has got a very clear strategy and message is that we want to see de-escalation, we're going to do everything we can to protect the UK diplomatic missions and we're going about that business.
He told the BBC's Andrew Marr programme that there "has not been a vacuum at all – the prime minister has been in charge."
Mr Johnson, who is expected back in Downing Street on Sunday, is under mounting pressure from opposition leaders to make a statement on the killing of General Soleimani.
Jeremy Corbyn, who wrote a letter to Mr Johnson requesting an urgent meeting , said: "Boris Johnson should have immediately cut short his holiday to deal with an issue that could have grave consequences for the UK and the world."
Acting Lib Dem co-leader Sir Ed Davey added to criticism of the PM.
"Johnson's silence on Trump's dangerous assassination in Iraq is deafening," Mr Davey said. "The Prime Minister must speak out now and make clear Britain will not support the US in repeating the mistake of the Iraq war."
Labour's John McDonnell vowed during an anti-war protest at Downing Street on Saturday to press Mr Johnson over the attack , which will "set the Middle East and the globe alight yet again".
"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," the shadow chancellor said.
A Government source defended Mr Johnson, saying "he's been kept fully up to date" including by Foreign Secretary Dominic Raab "at all times".
"And he will be meeting with ministers on Monday and speaking to foreign leaders over the next few days," they said.