Jeremy Corbyn has questioned whether the government has “credible evidence” to show Iran is behind the attacks on two oil tankers in the Gulf of Oman.
Foreign Secretary Jeremy Hunt said responsibility “almost certainly” lies with the Iranian regime.
But the Labour leader tweeted that the UK should ease tensions in the region, not fuel a military escalation.
The explosions on Thursday came in the Gulf of Oman – a strategic waterway crucial to global energy supplies.
It is the second time in the past few weeks that tankers appear to have been attacked in the region and comes amid escalating tension between Iran and the United States.
The US military released video footage which it said proved Iran was behind the blasts – something Iran has categorically denied.
The UK Foreign Office said it was “almost certain” that a branch of the Iranian military – the Islamic Revolutionary Guard Corps – attacked the two tankers on 13 June, adding that “no other state or non-state actor could plausibly have been responsible”.
“These latest attacks build on a pattern of destabilising Iranian behaviour and pose a serious danger to the region,” Mr Hunt said.
However, in a tweet Mr Corbyn questioned that assessment.
He previously cautioned against making “hasty judgements” in the wake of last year’s Salisbury nerve agent attack, which the government blamed on the Russian state.
His stance attracted some criticism, including from a number of his own MPs, although the Labour leader did subsequently say that the evidence clearly pointed to the Russian state.
US Secretary of State Mike Pompeo said Iran was behind what he described as the latest in a series of “unprovoked attacks”.
He said the US had made its assessment based on intelligence about the type of weapons used.
To back up its version of events, the US military released a video which it says shows Iran’s Revolutionary Guard removing an unexploded limpet mine from one of the ships – hours after the initial detonations.
The blasts came a month after four oil tankers were damaged in an attack off the coast of the United Arab Emirates. The US blamed Iran for that attack, but did not produce evidence. Iran also denied those accusations.
Tensions between the US and Iran have escalated significantly since US President Donald Trump took office in 2017.
He abandoned a nuclear deal that was brokered by Barack Obama’s administration and significantly tightened sanctions on Iran.