LONDON, Aug 5 (Reuters) -

Britain said on Monday it was joining a U.S.-led maritime security mission in the Gulf to protect merchant vessels travelling through the Strait of Hormuz.

Tanker traffic through the Strait has become a focus for an increasingly tense standoff between Washington and Tehran, into which Britain has also been dragged, and the United States 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.

"The UK is determined to ensure her shipping is protected from unlawful threats and for that reason we have today joined the new maritime security mission in the Gulf," Defense Minister Ben Wallace told reporters.

"We look forward to working alongside the U.S. and others to find an international solution to the problems in the Strait of Hormuz."

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

Earlier on Monday, Iran's Foreign Minister Mohammad Javad Zarif said Tehran would no longer tolerate "maritime offenses" in the strait.

It has threatened to block all exports via the Strait, through which a fifth of global oil traffic passes, if other countries comply with U.S. pressure to stop buying Iranian oil.

British foreign minister Dominic Raab said the latest move did not represent a change in approach to Iran and Britain remained committed to maintaining the 2015 nuclear deal agreed with Tehran in return for an easing of sanctions.

A British security 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.