A Spanish woman who has been living in the UK for the past 16 years and is married to a British national feared being unable to remain in the country in the long term because she did not take out health insurance when she was looking after her young son.

Maria Luisa Llorente, 45, discovered that EU citizens living in the UK are supposed to have private insurance when they are on parental leave or otherwise not working – and not to have taken it out could affect a person’s right to secure permanent residency in the country.

Under current rules, EU citizens who have lived in the UK for five consecutive years gain permanent residency rights provided certain criteria are met. For those in full-time employment this is rarely an issue. But for full-time mothers or fathers it can be.

Llorente lives in London and worked full time in the early part of the last decade but then took a break from work after having her son.

She returned to work part-time four years ago and after the EU referendum decided to apply for permanent residency. She then discovered that even though she was not dependent on the state, she had to have comprehensive sickness insurance in addition to proof that she was a permanent resident in the UK.

EU regulations require that EU citizens who are non-workers resident in another member state for more than three months must be covered by their own insurance. For workers there are reciprocal arrangements in place.

Llorente said she was concerned her application might not succeed, but that she had been told she was eligible for permanent residency because she spent six years working full-time between 2003 and 2009, and so was hopeful that she would be accepted.

But she believes her case – which has yet to be processed by the Home Office – nevertheless highlights the little-known regulation that would particularly affect EU citizens who are homemakers or carers living in Britain.

“I had never heard of this before and it is mainly affecting women, mothers, people who have to stop work or those that are sick or people who care for children or the disabled or elderly. It is very very unfair.

“I worked full-time between 2003 and 2009 and paid my national insurance and tax and just because you go on maternity leave you have to have this insurance. The problem is not so much having the insurance; it’s that we didn’t know about this and now it’s a retrospective requirement following Brexit. If I had known this is what I needed I would have got it,” said Llorente.

“It is not just people who stay at home who are being affected. I know one mother who is at home not out of choice. She has a child with respiratory problems, she’s a carer. These people are vulnerable and are really, really scared that at some stage they might not be able to justify their existence here.”

Another cohort affected are spouses of UK servicemen and women. One Italian woman who contacted the Guardian said she feared she may not qualify to remain because she spent most of her married life in Italy where her husband was posted and would not have proof of residency in the UK required to meet the five-year test should the couple seek to move back.