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The key Strait of Hormuz sits at the mouth of the Persian Gulf, a shipping channel for one-fifth of all global crude exports.The Canadian Press

Britain on Monday joined the United States in a maritime security mission in the Gulf to protect merchant vessels travelling through the Strait of Hormuz after Iran seized a British-flagged vessel.

British officials stressed that there was no change to London’s policy on Iran, but joining the United States is the most significant non-Brexit foreign policy move to date of Prime Minister Boris Johnson’s 12-day-old government.

Just two weeks ago, Britain was calling for a European-led naval mission. Now, it has joined what it said was a U.S.-led “international maritime security mission." No other countries are yet involved.

“It is vital to secure the freedom for all international shipping to navigate the Strait of Hormuz without delay, given the increased threat,” British Defense Secretary Ben Wallace said.

“The deployment of Royal Navy assets is a sign of our commitment to our U.K.-flagged vessels, and we look forward to working alongside the U.S. and others to find an international solution to the problems in the Strait of Hormuz.”

Tanker traffic through the Strait – through which a fifth of the world’s oil passes – has become the focus for a standoff between Iran and the U.S., which has beefed up its military presence in the Gulf since May.

Last month, Iran’s Revolutionary Guards seized a British tanker, Stena Impero, near the Strait of Hormuz for alleged marine violations. That came two weeks after Britain seized an Iranian oil tanker near Gibraltar, accusing it of violating sanctions on Syria.

Britain has repeatedly ruled out any exchange.

The tanker dispute has tangled Britain in the diplomatic differences between the European Union’s big powers – who want to preserve the Iran nuclear deal – and the United States, which has pushed for a tougher policy on Iran.

“Our approach to Iran hasn’t changed,” British Foreign Secretary Dominic Raab said. “We remain committed to working with Iran and our international partners to de-escalate the situation and maintain the nuclear deal.”

A British source said the focus of the new mission would be protecting the security of shipping and Britain would not be joining U.S. sanctions against Iran.

Another British source said London hoped the U.S.-led mission would transition toward a European-led mission. Britain has also offered to lead one of the U.S.-led mission’s maritime task groups.

No other countries have signed up to the U.S. mission.

Britain has deployed destroyer HMS Duncan and frigate HMS Montrose to the Gulf to accompany British-flagged vessels through the strait. So far, 47 ships have been accompanied by the naval vessels, British officials said.

Later this year, another Type 23 frigate, HMS Kent, will take over from HMS Duncan, while HMS Montrose completes planned maintenance. HMS Montrose will remain stationed in the Middle East until 2022.

Earlier on Monday, Iran’s Foreign Minister Mohammad Javad Zarif said Tehran would no longer tolerate “maritime offences” in the strait.

It has threatened to block all exports through the Strait if other countries comply with U.S. pressure to stop buying Iranian oil.

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