Shadow foreign secretary Emily Thornberry said it was important to establish independent evidence on who was behind the tanker attacks.
“These are extremely dangerous developments and we really have to pause and think about where we are going next,” she told BBC Radio 4’s Today programme.
“The idea that we are going to get enmeshed in another war is something that we really need to think about very carefully.”
There is the narrowest of differences in how the US and its closest ally, Britain, are ascribing blame over the tanker attack.
President Trump says “Iran did it”, while Jeremy Hunt says the Iranian regime was “almost certainly” behind it.
So is Britain blindly following the US into what could become a costly conflict?
Whitehall officials insist the evidence has been studied closely and they have reached the same conclusion as Washington: there are no other credible suspects apart from Iran.
It mined the entrance to the Gulf in the 1980s but strongly denies any role in this attack.
Yet a strange discrepancy has emerged with the owner of the Japanese tanker disputing the ship was hit with a limpet mine. Instead, he says, the crew reported “flying objects”.