LONDON — Britain warned Iran Monday that retaliatory fire on U.S. bases in the Middle East and the blockading of a key trade route would be a “catastrophic mistake.”

The Iranian parliament on Sunday endorsed a measure to close the Strait of Hormuz, a vital — and narrow — gateway for oil shipments from Persian Gulf countries.

The final decision, which will be made as Iran mulls its response to U.S. airstrikes on Iranian nuclear sites over the weekend, rests with Supreme Leader Ayatollah Ali Khamenei.

Speaking to the BBC Monday, Foreign Secretary David Lammy said he had urged his Iranian counterpart not to further escalate the conflict — and insisted that the supreme leader “gets” that it would be a mistake to blockade the strait.

“I was crystal clear to the Iranian [foreign minister]: it would be a huge, catastrophic mistake to fire at U.S. bases in the region at this time. We have forces in the region at this time,” Lammy told BBC Breakfast.

“It would be a catastrophic mistake. It would be a mistake to blockade the Strait of Hormuz. I think [Khamenei] gets that and understands that … I’m an optimist,” he added.

About 30 percent of global seaborne oil shipments pass through the Strait of Hormuz. U.S. Secretary of State Marco Rubio — who met Lammy in Washington last week — said Sunday it would be “economic suicide” to block the passage.

‘Return to the negotiating table’

Britain continues to push for deescalation in the wake of the U.S. strikes on three facilities aimed at preventing Iran from acquiring a nuclear bomb. Iran claims its nuclear ambitions are peaceful, though the International Atomic Energy Agency found Iran non-compliant with its nuclear obligations earlier June.

In a joint statement alongside France’s Emmanuel Macron and Germany’s Friedrich Merz, PM Keir Starmer called on Iran Sunday to “engage in negotiations leading to an agreement that addresses all concerns associated with its nuclear program.”

And in a further call with President Donald Trump Sunday evening, Starmer and Trump discussed “the need for Iran to return to the negotiating table as soon as possible and to make progress on a lasting settlement,” according to a Downing Street spokesperson.

While the U.K. was told in advance about Washington’s strikes on Iran, it wasn’t asked to take part — and ministers have refused to say whether Britain would get involved in any capacity if the U.S. falls into a deepening conflict with Iran.

Asked by Sky News whether the the U.K. was disappointed or pleased about the American strikes, Defense Minister Luke Pollard said it was “not for me” to comment on the U.S. action.

“I’m not going to be able to comment on that question, but what we can say is that we were not involved in the military action that the U.S. took,” Pollard said.