Foreign Secretary Dominic Raab has vowed to “leave no stone unturned” until British dual nationals arbitrarily detained in Iran are freed.

Mr Raab made the comments as he joined more than 55 countries in committing to enhance international cooperation to end the “cruel” practice of arbitrary detention.

He said the practice violates the human rights of individuals and undermines trust in institutions “that keep our societies safe and open”.

In pre-recorded remarks to the Canadian initiative on Arbitrary Detention, he said: “Countries around the world must work together to counter the unacceptable use of arbitrary detention of foreign and dual nationals, used to gain diplomatic leverage over their governments.

“Arbitrary detention doesn’t just violate the human rights of individuals. It undermines trust in the institutions that keep our societies safe and open. The use of arbitrary detention, particularly as a threat, or as a means to influence others, is cruel. It’s wrong.

“We continue, for example, to press for the immediate and permanent release of all arbitrarily detained dual British nationals in Iran. I will leave no stone unturned until this is done.”

British-Iranian dual national Nazanin Zaghari-Ratcliffe has been detained in Iran since 2016, when she was sentenced to five years in prison over allegations, which she denies, of plotting to overthrow the Iranian government.

He said the countries together “call on all states to end the practice of arbitrary detention of foreign and dual nationals”.

Mr Raab added: “And we call on states to abide by their clear obligations, including under the Vienna Convention on Consular Relations – including Iran, China and many others.”

The declaration has been endorsed by the European Union, United States, Australia and others.