The group has asked the new Prime Minister to meet with them to discuss “what he can do to guarantee the right to consular protection in UK law”.
Their letter, which claims 2,000 British citizens are currently being arbitrarily held overseas, continues: “The British Government and institutions are inconsistent in their action towards defending British citizens’ rights abroad.
“Many of them remain vulnerable to the unlawful and inhumane treatment suffered in other states, both friends and foes. This cannot continue.”
Along with Mr Ratcliffe, it is signed by relatives of Matthew Hedges, the academic who last year spent seven months detained in the UAE; Andragachew Tsege, a dual-nationality British-Ethiopian who spent years in an African jail; Billy Irving, one of the so-called Chennai Six jailed in India; and Jagtar Singh Johal, a Scottish sikh who is still being held by Indian authorities.
Mr Ratcliffe told the Telegraph it took a “long time” for the Government to firstly acknowledge his wife’s plight and then to confront Iran about it.
He said: “You and I have got more rights if we fall over on the pavement than we have if we are held a prisoner abroad and that just feels wrong.
“You need your Government’s protection when you’re most vulnerable – which means when you’re subject to torture or subject to completely unfair imprisonment.”
He added: “There is definitely a protection gap. We in our family hope that other people don’t have to go through what we are going through.” His views were echoed by Daniela Tejada, Mr Hedges’s wife, who believes he could have been released earlier if consular protection was provided at the start of his case.
She said: “I think it would have, at the very least, ensured he wasn’t mistreated and tortured for so many months.
“Our struggle doesn’t end the moment our loved one is released, it is merely the beginning of it because they are scarred by the trauma of their experience.
“That is what we are trying to avoid at a fundamental level.”
Mr Hedges has revealed he is suffering from PTSD after being falsely accused of spying for MI6 and spending months in solitary confinement in a windowless cell.
A petition has also been started by the group to promote their campaign.