British Prime Minister, Boris Johnson

Britain has 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 nations 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,” said British Defence Secretary Ben Wallace.

“The deployment of Royal Navy assets is a sign of our commitment to our UK flagged vessels and we look forward to working alongside the US 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 United States, 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.