O’Connor, who moved to the UK aged six, had been told she was an illegal immigrant

One of the most prominent victims of the Windrush scandal, who spoke in parliament earlier this year to describe the severity of the problems she was facing, has died aged 57.

Sarah O’Connor was found dead at her home on Sunday morning. A postmortem will be held on Friday, but her death has provisionally been attributed to natural causes.

In March, O’Connor told the Guardian she was facing bankruptcy as a result of being classified as an illegal immigrant. At the start of this year, she was so worried about her predicament that she said she was afraid to open the front door for fear that it might be the Home Office coming to to deport her, or bailiffs arriving to remove her possessions.

She moved to Britain 51 years ago, but had struggled to make officials understand she was in the country legally. As a result, O’Connor was unable to take up a new job and was refused unemployment benefits, leaving her without an income. She spent the final year of her life trying to extract herself from a spiral of problems that emerged because of an official decision to categorise her as someone who was in the country illegally.

Although O’Connor went through a naturalisation ceremony at the end of July and was formally recognised as British, her problems continued. In the weeks before she died, her landlord had given her notice to leave and, still struggling to get work, she was having difficulty finding a new home.

Play Video 3:10 Windrush citizens: 'It's like having your world torn apart' – video

Her daughter, Stephanie O’Connor, said her mother had been relieved to have been through the naturalisation ceremony, but also annoyed it had been necessary. “She was so happy to have finally got somewhere; we’d been banging our heads against the wall for so long. But she had mixed emotions about it – she felt she should always have had it,” her daughter said.

Stephanie O’Connor said the immigration problems, which emerged a year ago, had badly affected her mother. O’Connor had previously been “very bubbly, always laughing, someone who saw the positive”.

“It made her very unhappy. I saw a complete change in her,” her daughter said. “She wasn’t the same mum any more. She felt like she wasn’t getting anywhere, and she was deflated. I was trying to keep her upbeat; she said she just wanted to give up.”

Facebook Twitter Pinterest Sarah O’Connor, left, Anthony Bryan, Paulette Wilson, Sylvester Marshall and Elwaldo Romeo outside the Houses of Parliament in May. Photograph: Yui Mok/PA

O’Connor was extremely angry at the treatment she received at the hands of the Home Office. She had lived in England for more than half a century, been to primary and secondary school in London, had a driving licence, voted in general elections, been married for 17 years to a British citizen, and had four children in the UK, all of whom have British passports.

She had worked continuously, at times for Ford in Dagenham and in retail, paying taxes and national insurance contributions. She had never been on holiday abroad, so had not seen the need to apply for a British passport. O’Connor could not understand why her immigration status was being questioned.

She had been forced to sell her car and ask her daughter for financial help. “It was hard for her to pay the rent and buy food. She hated relying on me,” Stephanie O’Connor said.

When it became clear that thousands of people, who were born in Commonwealth countries and came to the UK as children in the 1950s and 1960s, were similarly affected, O’Connor was determined to campaign for change and spoke several times on television about what happened. “She wanted to help other people in her situation. She wanted to raise awareness,” her daughter said. The activism reenergised her. “I felt like the old mum was coming back.”

She spoke at an event in parliament in May attended by the immigration minister, Caroline Nokes, as well as Jeremy Corbyn and Diane Abbott, explaining how devastated she was to be told she was an illegal immigrant.

“I’ve been here for 51 years, I’ve worked for 30-odd years and got told that I’m an immigrant, I’m not entitled to anything,” O’Connor told the meeting.

“When all this happened to me and I was told I was an immigrant, I wouldn’t cry in front of the jobcentre, I went home and broke down. I was only six when I came over here. How was I supposed to know all of these laws? It just breaks me.”

She told them it had reminded her of the racism she experienced in the 1970s. “I grew up when the National Front was about, and that was a really bad time,” O’Connor said.

“As the years went on, I thought that had been stamped out, but reading everyone’s stories, it feels like it has become a hostile country again. It’s all wrong.

“The government and parliament need to sit down and sort something out. Aren’t we all British? Why do we have to go through all this rigmarole and hardship and pain?”

Omar Khan, the director of the Runnymede Trust, a race equality thinktank, tweeted: “Very sad to hear Sarah O’Connor has passed away.

“No one who heard her will forget her speaking out in parliament, urging others affected by the Windrush injustice to raise their voice, and for all of us to tackle racism and injustice.”

Her MP, Labour’s Margaret Hodge, said: “She had been through hell. She was forced to sell her clothes and her car. She showed a really positive, brave resilience in the face of terrible circumstances.”