Her continued imprisonment has worsened already-strained relations between the two countries.
Diplomatic protection is a rarely-used mechanism through which a state can seek protection on behalf of one of its nationals if it believes they have been wronged by another state.
The UK has not used it in recent memory.
Mrs Zaghari-Ratcliffe, who had been working for the Thomson Reuters Foundation, was arrested in April 2016 at Tehran airport when she and her then 22-month-old daughter, Gabriella, were about to return to the UK after a family visit.
During her subsequent trial, she was accused of running “a BBC Persian online journalism course” and seeking a “soft overthrow” of Iran.
Human rights lawyers have previously said the “grave harm” she has suffered during her detention at Evin prison made her eligible for diplomatic protection.
The United Nations Working Group on Arbitrary Detention said Iran’s refusal to provide treatment for Mrs Zaghari-Ratcliffe, who is complaining of lumps in her breasts, was “arbitrary and unlawful” and may amount to torture.