Theresa May and her husband Philip arrive at the annual Lord Mayor's Banquet in Guildhall: she is considering taking strong diplomatic action against Iran

Theresa May is considering whether to invoke a 250-year-old legal principle that would escalate the imprisonment of a British mother in an Iranian jail into a full diplomatic crisis.

Nazanin Zaghari-Ratcliffe, 38, may be given diplomatic protection status, according to No 10 yesterday, which would make her detention an act of aggression by Tehran against the British state, rather than against an individual. The law has never been invoked in the UK but has been used by other countries, including Germany.

The detained charity administrator’s husband asked Redress, the human rights group, to write a legal opinion on the matter, which was given to the Foreign Office in September. Richard Ratcliffe raised the issue with Boris Johnson, the foreign secretary, in a phone call on