Ministers have been accused of breaching their promise to secure the post-Brexit rights of thousands of British nationals who settled in the EU and married foreigners.

Campaigners at British in Europe (BiE) have written to the Foreign Office minister Wendy Morton and the immigration minister Kevin Foster telling of the “heartbreak” and “distress” endured by British citizens who are facing problems with the Home Office over their rights to return home to the UK.

Before Brexit, all EU nationals – including the British – were free to move from one member state to another with family members, regardless of where they were from.

But after Brexit, UK citizens’ non-British spouses – including Germans, French and Spanish nationals – need to apply for pre-settled status before 29 March next year. However, they are only eligible if they get a new EU family permit from the Home Office.

BiE said it knows of multiple cases of spouses of British people living in the EU being refused family permits under the “Surinder Singh route” for non-British spouses or partners or who are facing “significant problems” with their applications.

It told Morton the examples were “precisely the kinds of situations we knew British citizens and their families would face following the loss of our EU rights and why we asked for a grace period”.

“During the passage of the immigration bill, your ministerial colleagues Kevin Foster and Baroness Trafford gave assurances to other MPs and peers that such families … futures’ were safe. Quite clearly this is not the case,” said the letter.

BiE is calling on the government to extend the deadline for applications beyond 29 March 2022 because of the delays to give British people the chance to exercise their rights.

One heavily pregnant British woman living in Barcelona said she could not come home to visit her father, who has recently suffered a brain injury, after her husband was refused a permit. Olivia Hughes said her Moroccan husband, Abdel, a legal Spanish resident, had been refused a permit because of paperwork and was now on a knife-edge over a potential appeal which may force her into a first tribunal court case.

Hughes, a primary school teacher, said she was shocked and distressed at the “harsh treatment” and “lack of compassion” of her own government.

“You have to apply on the government website and send in a lot of documentation, but it’s really unclear exactly which forms of evidence you need to provide for them to accept,” she said. “They could have just asked for the extra paperwork but instead refused. An appeal could take months or a year and then we miss the deadline for pre-settled status.”

She has written to the MP for Hove, Peter Kyle, asking him to take her case up.

“The sad thing is it is making problems between us. To not be able to come back with my husband when an emergency happens is just really difficult to swallow. And we need to move over together because, at seven months pregnant, I do not want to risk being on my own. It is just heartbreaking,” she said.

“We are not fake. This is literally tearing my family apart and is currently having extremely detrimental effects on my father’s health and my own mental health.”

Hughes said the Home Office could have treated British nationals like EU nationals, and simply given them a chance to provide supplementary evidence rather than force them into a lengthy tribunal process – which could ultimately mean time runs out on their right to come home.

Fiona Godfrey, the chair of BiE, said Olivia’s case was typical of Britons trapped in a post-Brexit nightmare simply because they fell in love with a non-British national.

Another woman, Jenny Gruner, also living in Spain, now fears that her application to return home with her German husband, Christian, could time out or fail because of the Home Office delays.

“If we don’t get this family permit on time, I know we will never be able to go back to the UK as a family because I am never going to be able to afford the spouse visa fees – which are GBP8,000,” she said.

Gruner, from Hackney in east London, said: “I want to move to Kent to be closer to my sister. My little boy is three years old and we need to pick a school by December. It is so unfair. The deadline for applications is so tight after Brexit.”

Another woman, married to a British man for six years, with their own business and mortgage-free home in the Czech Republic, said they had applied to the Home Office in January for a permit and were told they would have to wait two weeks. They are still waiting. “We find it appalling that our EU friends can stay in the UK with their non-EU spouse yet I don’t even have the permission to enter the country,” she said.

The Home Office said: “British citizens returning to the UK with their family members from the EU should apply for an EU settlement scheme family permit.

“Each case is considered as quickly as possible and on its individual merits.”