Lawyers for Nazanin Zaghari-Ratcliffe have been told by the Foreign, Commonwealth and Development Office (FCDO) that the British government "is not a statutory safeguarding body, nor does it have a legal duty of care to British nationals overseas".
Senior MPs, including former Foreign Secretary Jeremy Hunt, have criticised the FCDO's assertion.
Daughters of Iranian prisoners want parents home for Christm
Writing to Nazanin's lawyers, Sarah Broughton, the FCDO's head of consular services, said there was no legal right to protection and that all cases, including Nazanin's, were treated on a discretionary basis.
She added: "We can assure you that Mrs Zaghari-Ratcliffe’s welfare and safe return to the UK remains a priority for the FCDO and we continue to provide her and her family with our utmost assistance, at all times acting in ways that we believe are in her best interests."
The FCDO reiterated it does not consider Nazanin's detention as "hostage-taking".
Last week Richard welcomed a strongly-worded report from MPs calling for the government to start describing Nazanin and others like her hostages. She has been told in private that her imprisonment is connected to the UK's unpaid £400m debt to Iran.
The family's legal team responded with a further letter just before Christmas, asking the FCDO whether it considered it the UK's responsibility or Nazanin's family's to bring her home, and asking again whether it considered it did not have any legal responsibility to protect her from torture.
Nazanin was first arrested in Tehran in 2016, and eventually convicted of spying. She has always denied this, but a second legal case is hanging over her.
Writing in the Times, Jeremy Hunt said: "We must show the world that if you imprison a British citizen on trumped-up charges you will pay a very heavy price because Britain is a major player on the world stage and intends to remain one."
A FCDO spokesperson said the UK government was "working tirelessly" to support British citizens around the world, and added: "We are doing all we can to help secure the permanent and immediate release of Nazanin and all British dual nationals arbitrarily detained in Iran."
They said the UK was discussing concerns over how to raise cases of arbitrary detention with a number of other countries.
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