Iran's seizure of a British-flagged tanker raises very serious questions for British and international shipping in the Strait of Hormuz, foreign minister Jeremy Hunt said on Saturday.

Stena Bulk, the Swedish-based owners of the UK-flagged tanker detained by Iran, said on Saturday they were preparing a formal request to visit its 23 crew members.

British foreign minister Jeremy Hunt said he had spoken to his Iranian counterpart Mohammad Javad Zarif and expressed his "extreme disappointment" over Iran's seizure of a British-flagged tanker in the Strait of Hormuz on Friday.

Britain summoned the Iranian Charge d'Affaires in London after Friday's seizure of a British-flagged tanker in the Strait of Hormuz, the British Foreign Office said on Saturday. (Reporting by Stephen Addison Editing by Peter Graff)

British Foreign Secretary Jeremy Hunt said London would respond in a considered but robust way to the seizure of a British tanker by Iranian guards, and warned that Tehran would be the biggest loser if freedom of navigation is restricted.

* Map showing Stena Impero location https://tmsnrt.rs/32EB5Yv (Adds Hunt's later comments reported by Sky News, link to oil report)

British Foreign Secretary Jeremy Hunt said on Friday there would be consequences if Iran did not return control of ships seized in the Strait of Hormuz, Sky News reported.

Oil tanker Mesdar, which had turned in the direction of Iran, was boarded by armed personnel at approximately 1630 GMT but they have now left and the vessel has resumed its voyage, its British operator said on Friday.

Britain said Iran's seizure of a British-flagged vessel and a Liberian-flagged vessel in the Strait of Hormuz was unacceptable and called for freedom of navigation in the Gulf.

Britain said Iran's seizure of a British-flagged vessel and a Liberian-flagged vessel in the Strait of Hormuz was unacceptable and called for freedom of navigation in the Gulf.