Lindsay and Craig Foreman, from East Sussex, were detained in January while on a motorcycle tour around the world and later charged with espionage, allegations they strongly deny.

Ms Foreman’s son, Joe Bennett, told Sky News that the family has received consular support from the Foreign Office and has met Foreign Secretary Yvette Cooper, but he described growing frustration with the government’s approach.

He said officials appeared to be waiting for Iran’s judicial process to conclude, despite long delays and a lack of clarity.

Mr Bennett said his parents were told nearly two months ago that sentencing would take place within weeks, but no decision has been made.

He criticised what he sees as a reluctance by the government to publicly challenge the charges, despite their seriousness.

“They are accused of espionage, spying for the British state, and yet that same UK government won’t come out and defend them,” he said, adding that the charges are “state-level” and demand a stronger response.

Mr Bennett said his mother is “suffering a lot with her skin and weight” owing to a lack of ventilation and poor conditions in the prison.

He said the couple are living in dirty conditions, with rats in cooking areas.

Mr Bennett said he feels “a sense of guilt” while his mother is “stuck in a cell,” unable to celebrate Christmas.

The Foreign Office advises against all travel to Iran due to the risk of detention.

In a statement in October, it said it remains “deeply concerned” about the case, is providing consular assistance and continues to raise the matter with Iranian authorities.