Iran's seizure of a British-flagged oil tanker in the Strait of Hormuz was a "legal measure," the spokesman for the Iranian government said on Monday.

"Seizing the British tanker was a legal measure by Iran. Iran confronted the ship (to ensure) the region's security," Ali Rabiei told a news conference in Tehran.

"To all the countries that are calling on Iran to release the tanker, we ask them to tell Britain the same thing," Rabiei said, referring to the seizure by the British authorities of an Iranian tanker in the Mediterranean on July 4.

Britain was weighing its next moves in the Gulf tanker crisis on Sunday, with few good options apparent as a recording emerged showing that the Iranian military defied a British warship when it boarded and seized a ship three days ago.

Prime Minister Theresa May's office said she would chair a meeting of Britain's COBR emergency response committee on Monday morning to discuss the crisis.

Britain has given little clue as to how it plans to respond after Islamic Revolutionary Guard Corps (IRGC) rappelled from helicopters and seized the Stena Impero in the Strait of Hormuz on Friday in apparent retaliation for the British capture of an Iranian tanker two weeks earlier.

According to Reuters, on Sunday the tanker was docked in Bandar Abbas port with Iran's flag now hoisted atop.

The British government is expected to announce its next steps in a speech to parliament on Monday. But experts on the region say there are few obvious steps London can take at a time when the United States has already imposed the maximum possible economic sanctions, banning all Iranian oil exports worldwide.

"We rant and rave and we shout at the ambassador and we hope it all goes away," said Tim Ripley, a British defense expert who writes about the Gulf for Jane's Defense Weekly.

"I don't see at this point in time us being able to offer a concession that can resolve the crisis. Providing security and escort for future ships is a different matter."

A day after calling the Iranian action a "hostile act," top British officials kept comparatively quiet on Sunday, making clear that they had yet to settle on a response.

(With input from Reuters, AFP)