Tehran have seized two shipping vessels in the Strait of Hormuz and the Foreign Secretary says he is "extremely concerned".
Jeremy Hunt added: "These seizures are unacceptable. It is essential that freedom of navigation is maintained and that all ships can move safely and freely in the region."
Iranian state television said it was seized because the tankers 'failed to observe international maritime regulations'.
Sky's foreign affairs Editor Deborah Haynes described the situation as British officials' worst nightmare for an outgoing Theresa May.