Jeremy Corbyn has told the Government not to blame Iran for the attacks on two oil tankers without giving "credible evidence".
Foreign Secretary Jeremy Hunt supported the US government's claim Tehran was behind the attack, saying it was "almost certain" in its assessment that a "branch of the Iranian military… attacked the two tankers on 13 June”.
Donald Trump himself came out and said the attack "had Iran written all over it".
The US military released video footage which it said showed Iran’s Revolutionary Guard removing an unexploded limpet mine from one of the oil tankers targeted near the Strait of Hormuz.
The Foreign and Commonwealth Office said "no other state or non-state actor could plausibly have been responsible", adding there was a “recent precedent for attacks by Iran against oil tankers”.
However Mr Corbyn said Britain's support of the US claims risked escalating the threat of war in the Gulf.
Mr Corbyn tweeted: “Britain should act to ease tensions in the Gulf, not fuel a military escalation that began with US withdrawal from the Iran nuclear agreement.
“Without credible evidence about the tanker attacks, the government’s rhetoric will only increase the threat of war.”
Shadow Chancellor John McDonnell supported Mr Corbyn's view that assessments should not be made until an independent panel had confirmed the US accusations.
He said: "We must not be in a situation where we escalate the risk, which is the risk of war in that region, unless there is sound evidence."
He added: "The reality is this: haven't we learnt the lessons about interventions in the Middle East from Iraq? That you do not do it on the basis of what is evidence from the US that so far has not been fully tested, and certainly has not been made available to Her Majesty's opposition."
Mr Hunt defended the US assessment, saying: "These latest attacks build on a pattern of destabilising Iranian behaviour and pose a serious danger to the region.
“We call on Iran urgently to cease all forms of destabilising activity. The UK remains in close coordination with international partners to find diplomatic solutions to de-escalate tensions.”
The Foreign Secretary told ITV News' Political Correspondent Paul Brand, on Saturday: "You'd think it'd be pretty obvious who was responsible for this when we actually have video evidence that shows what the Iranians have been doing.
"But oh no, for Jeremy Corbyn it's all America's fault - and this is the same man, by the way, that refused to condemn Putin after the Salisbury Novichok attacks and I'm afraid this shows that Labour is in the grip of virulent anti-Americanism that will be incredibly dangerous for our country if they ever took control."
Iran has denied involvement in the attacks. President Hassan Rouhani said the US was carrying out an aggressive policy and posing a serious threat to regional stability”.
Tehran’s foreign minister, Javad Zarif, earlier said the US “immediately jumped to make allegations against Iran without a shred of factual or circumstantial evidence”.
On Friday, Donald Trump told Fox News: “I guess one of the mines didn’t explode and it’s probably got essentially Iran written all over it. And you saw the boat at night trying to take the mine off and successfully took the mine off the boat, and that was exposed.”
The price of oil rocketed on Thursday amid fears of disruption to one of the world’s most important tanker routes as a result of instability in the region.
- Video report by ITV News Washington Correspondent Robert Moore
The suspected attacks came after four commercial ships were “subjected to sabotage operations” in the Arabian Gulf in May.
The FCO said a UAE-led investigation concluded that they were conducted by “a sophisticated state actor”.
“We are confident that Iran bears responsibility for that attack,” the FCO said.