A number of long-term British citizens have expressed alarm at receiving letters from the Home Office telling them they risk losing the right to work, benefits and free healthcare unless they apply for UK immigration status in the next six weeks.

Campaigners said they were concerned that the “scattergun” mailshot, which was sent out to thousands of people instructing them to apply for EU settled status before the end of June, revealed weaknesses in the Home Office’s databases, and a lack of bureaucratic clarity about who has the right to live in the UK.

The letter, which was wrongly sent to numerous people who have lived in the UK for over 40 years, states: “The United Kingdom has left the European Union, so to carry on living in the UK after 30 June 2021, you and your family members need to have a UK immigration status,” the letter states.

Among those who received the letter were several people with dual citizenship, including retired nurse Marianne Howard, 82, originally from Germany, who has been a British citizen for over 50 years; retired structural engineer George Smid, who was born in what was then Czechoslovakia and became a British citizen in 1987; Isabella Moore, originally from Poland, who has had citizenship for over 40 years and spent 33 years working as an NHS doctor; academic Jan Culik, who holds a Czech passport and naturalised in the UK 36 years ago; and architect Eva Apollo-Crawshaw, originally from Poland, who has been a British citizen for 40 years.

Several expressed their unease about the insensitivity of the letter’s wording, which highlighted that urgent action was required if the recipient and their family were to continue to be eligible for benefits, free healthcare and the right to work in the UK. Some, but not all, versions of the letter include a paragraph on the second page, telling recipients to ignore the notification if they already have citizenship. Anyone confused by the letter was invited to call a helpline, and several of those who tried to do so over the weekend and on Monday were directed to an answerphone message which stated: “We are experiencing a high demand for our services and currently have no more space in our call queue.”

Some recipients said they were disturbed to discover that they remain classified as foreigners on internal Home Office databases, despite having been British for decades.

Helen Howard-Betts said her mother, Marianne Howard, who married a British citizen in 1965 and who has two British children and four British grandchildren, “went into a complete panic” when she received the letter. “It seems like an algorithm has done this. She has heart disease and her blood pressure went through the roof. I can’t understand why they don’t cross-check the databases to see if someone already has permission to stay,” she said. “Why create unnecessary stress by sending a letter like that to people to whom it doesn’t apply?”

Maike Bohn, co-founder of the3million, a grassroots organisation supporting EU citizens in the UK, said: “It is concerning that the Home Office sends frightening letters urging people to apply to the EU settlement scheme to remain eligible for benefits, and are not able to exclude those who have naturalised to become British decades ago or who already hold pre- or settled status.”

One woman, who asked for her name not to be printed, said she had received two similar letters in the space of a week, informing her that she needed to apply for settled status, despite the fact that she had applied for it and been granted it last year, having lived in the UK for 14 years. “They should have a better system and not scare people like this. It is irritating because after Brexit we feel like second-class citizens anyway,” she said.

Isabella Moore, a consultant pathologist who highlighted the problem in a letter to the Guardian said: “I’ve had British citizenship for the past 40 years. I don’t understand why I’m being singled out as a foreigner and told I’m getting benefits from the Department for Work and Pensions. I paid national insurance contributions for 33 years.” George Smid said he was worried that the letter mentioned his family’s eligibility: “I felt complete disbelief. My daughters were born here and are in their 30s.”

Green party peer and former party leader Natalie Bennett said: “That these letters are being sent shows a very serious failure of government systems. Massive stress is being caused to UK citizens, many of them elderly, who are falling foul of the hostile environment. This is dreadfully reminiscent of the Windrush scandal, and demonstrates a ‘couldn’t care less’ attitude in government.”

A Home Office spokesperson said: “The government is using every possible channel to encourage anyone who may need to and is eligible for the EU settlement scheme to apply.

“In trying to reach as many people as possible there may be a small number of cases where letters have been sent to someone who is naturalised as a British citizen or has already applied to the EUSS. In these cases the letter makes clear that no action is required.

“We would urge anyone eligible for the EUSS to apply before the 30 June deadline. Already there have been 4.9m grants of status.”