Iran’s ambassador to the UK has been summoned to the Foreign Office (FCO), following the detention of his British counterpart in Tehran last week.
The FCO said the arrest of Rob Macaire after a vigil for victims of last week’s plane crash was illegal and should be investigated.
Foreign Secretary Dominic Raab said the UK would “demand an apology” and seek assurances it will not happen again.
Iranian ambassador Hamid Baeidinejad is expected to attend the FCO later.
Mr Macaire was attending an event on Saturday, which was advertised as a vigil for the 176 people who died in Wednesday’s crash of an Ukraine International Airlines Boeing 737, shortly before he was arrested.
He was held for three hours when he stopped at a barber shop for a haircut on his way back to the British embassy.
