An investigation into the causes of the Windrush scandal has been completed and presented to the UK home secretary, who plans to make it public on Thursday.

The investigation is expected to be very critical of Home Office staff and successive government ministers, and is likely to focus on the “hostile environment” immigration policy introduced by Theresa May when she was home secretary, and the role of institutionalised racial discrimination.

The Windrush Lessons Learned review was commissioned by Sajid Javid as home secretary weeks after the scandal came to public attention following a series of articles in the Guardian in April 2018.

Wendy Williams, an inspector of constabulary, has been working on the report for about 20 months. She had been expected to submit it in the autumn but its completion was delayed by a process known as Maxwellisation, which allows for any individuals criticised in an official report to respond before its publication.

The scandal arose after the Home Office wrongly designated thousands of legal UK residents as being in the country illegally. Some were mistakenly deported to countries they had left as children about 50 years earlier, and others were wrongly detained in immigration removal centres. Many lost their jobs after being told they did not have the right to work in the UK and were later denied benefits, leaving them destitute. Many were made homeless, denied NHS treatment and prevented from travelling.

Leaked versions of an early draft of the report stated that the Home Office had failed in its legal duty to counter racial discrimination when it implemented its anti-immigration “hostile environment” programme. That draft accused officials of recklessness, defensiveness and a reluctance to acknowledge and learn from their mistakes. However, the final report is likely to have evolved considerably from the earlier version leaked in 2019.

May introduced a series of “hostile environment” policies under which she sought to make life intolerable for people who had entered the UK illegally, in an effort to cut immigration numbers during her time as home secretary.

The Home Office was warned that these policies would adversely affect many people who were living here entirely legally, but officials appear not to have paid sufficient attention to those warnings. Williams’s review focuses on the impact of immigration laws from that period.

Williams urged the current home secretary, Priti Patel, to publish the review as soon as possible.

A Home Office spokesperson said: “We expect to publish the report and its findings tomorrow, subject to parliamentary time.”

There has been concern from some of those affected by the scandal that the report may not get the attention it needs if it is published during this period of national crisis.

Winston Robinson, who arrived from Jamaica aged nine in 1966, was sacked from his job as an ambulance driver in 2016 because he had no passport, and later became homeless. He said: “I hope that the report recognises that people’s lives were turned upside down, and accepts the damage that was done and the mistakes that were made. I’m worried that it is going to be put on the backburner because of everything else that’s going on.”

Michael Braithwaite, 68, arrived with his family from Barbados in 1961, aged eight. He lost his job as a special needs teaching assistant after the primary school where he had worked for more than 15 years deemed him an illegal immigrant, despite the fact he had lived in the UK for more than 50 years. He said he hoped the report reflected the magnitude of what went wrong. “People who did nothing wrong died; others are still destitute. I’m still suffering, wondering: am I really safe here?”

Since the scandal emerged, more than 8,000 people have been given documentation proving that they are – and always were – living legally in the UK. A compensation scheme has been set up with an estimated budget of between £200m and £570m, but at the last count only £62,198 of compensation had been paid out, shared between 36 people.

A Home Office spokesperson said: “We are grateful to Wendy Williams for submitting her independent review to the Home Secretary.

“We expect to publish the report and its findings tomorrow, subject to parliamentary time.”