The Home Office is trying to separate a couple from their four British children, 11 grandchildren and a great-grandchild by forcing them to return to Iran.

The 83-year-old great-grandfather and 73-year-old great-grandmother, who bought their flat in Edinburgh in 1978, live near their close-knit family and depend heavily on their daily support. But they also act as co-parents to one of their grandchildren, a teenager with severe autism who requires constant supervision. Their help enables the boy’s mother – a single parent – to continue her work as an NHS nurse.

Separation of the teenager from his grandparents, Mozaffar Saberi and Rezvan Habibimarand, would be extremely detrimental to him and also to his mother, the couple’s daughter, according to a chartered psychologist with expertise in children with autism, who has written a report provided to the Home Office.

“Going back to Iran would be the end for us,” said Habibimarand. “We have so many illnesses that it would not just be physically the end for us, because there is not the level of healthcare we need in Iran, but emotionally the end too: there’s no one in Iran for us to go back to.”

Navid Saberi, the couple’s son, said: “With no exaggeration, sending them back to Iran would be a death sentence. The day-to-day help and support my siblings and I give our parents isn’t available to be purchased in Iran, even if we could somehow get the necessary money into the country – which is not at all guaranteed because of the sanctions. The distress of having to live alone would mean my parents’ end would come very soon.”

The couple have made repeated human rights applications to remain in the UK since 2013 but the Home Office has refused all of them. Their final appeal is due to be heard soon.

John Vassiliou, a partner at McGill & Co Solicitors, said: “The Home Office does not give any weight to the relationships with their adult children and contrary to the conclusions of the independent expert, and without so much as an interview with any member of the family, took the view that their autistic grandson could adapt to their absence.

“They also said that the child’s mother ‘can seek assistance from social services who can provide specialist care for her son’. It seems that the Home Office would prefer that someone quit their job and resort to burdening the public purse rather than allowing the child’s grandparents to stay and help out.”

Although the couple raised their children in the UK, they never sought British citizenship. But because they originally came to the UK as visitors and then made a human rights claim to remain, it is highly likely that – even if their health allowed it – any future visit visa would be refused.

“The consequence is that they would be unable to see their family again unless the family all travelled to Iran to see them,” Vassiliou said. “That comes with its own problems, especially for their autistic grandson.”

Prior to July 2012 – just nine months before the couple made their application – an adult relative’s acute need of emotional, physical or even financial support from their UK-based adult children would have been enough for them to be granted visas. But the then home secretary, Theresa May, introduced sweeping changes to the immigration rules to reduce net migration” to the UK, which included a tightening of what is known as the “adult dependant relative” rule.

“The consequence of this tightening is that it has become almost impossible for any British citizen or settled person to bring their adult parents in to the UK to live with them,” Vassiliou said. “The criteria that have to be satisfied are so severe that I have yet to meet anyone who has been able to meet them.”

The couple’s upcoming appeal is based on the claim that the Home Office has failed to properly take into consideration the best interests of the autistic child, has never spoken to any member of the family and has found grounds for refusal by equating the couple’s faltering health due to old age with an inability to act as co-parents to the child on an emotional level.

The refusal letter, Vassiliou said, makes “bewildering statements”, such as: “It is noted that you own the house you reside in Edinburgh, therefore you could choose to allow your daughter and grandson to live there on your return to Iran, which then would not impact on your grandson as you claim he visits you there every day.”

Vassiliou added: “In all of this, the government seems to have completely lost sight of the fact that these are two very elderly people with nobody in Iran, and with an entire family spanning three generations in the UK. It seems inhumane to us that such a matter is even up for debate.”

A spokesperson for the Home Office said: “All UK visa applications are considered on their individual merits, on the basis of the evidence available and in line with UK immigration rules.”