Wholesale reform of a “reckless” and “defensive” Home Office is expected to be recommended in a hard-hitting review into the causes of the Windrush scandal when it is released by the home secretary on Thursday.

The Windrush Lessons Learned review is expected to criticise Home Office staff and government ministers for their continued failure to admit the magnitude of their mistakes and the scale of damage inflicted on thousands of legal UK residents who were wrongly classified as illegal immigrants, with catastrophic results.

Those involved in preparing the long-awaited report, which is understood to run to several hundred pages, were dismayed at the timing of the publication, at a moment when the attention of both politicians and the public is entirely preoccupied with the coronavirus emergency.

The report was submitted to the Home Office by its author, Wendy Williams, an inspector of constabulary, on Wednesday, after 20 months of work.

The home secretary, Priti Patel, decided to release it very swiftly and is due to make a statement to the House of Commons on the review and its recommendations on Thursday morning.

A leaked early version of the report seen by the Guardian indicated that the true scale of the scandal may still remain hidden because many people affected have so little trust in the Home Office that they are scared to come forward.

The Guardian has reported on numerous cases of people who remain homeless, almost two years after the government first apologised for its mistakes and promised to put things right.

The report is likely to focus on the discriminatory effects of “hostile environment” immigration policies introduced by Theresa May when she was home secretary, and to condemn Home Office staff for failing to treat those affected with dignity or adequate respect. The Home Office is likely to be criticised for failing in its duty to counter racial discrimination.

Williams’s review is expected to highlight the “long-standing tension” between the Home Office’s duty “to protect vulnerable people and communities” and its parallel goal of controlling migration. It may also criticise a “negative culture” in the Home Office and stress the need for robust whistleblowing policies, and for junior staff to feel able to speak up when they see things going wrong.

The report is also expected to argue that Windrush was able to happen because officials had a “poor understanding” of Britain’s colonial history, the history of migration to the UK and the history of black Britons, and will recommend that the department better reflects the society it represents.

The scandal arose after the Home Office wrongly designated thousands of legal UK residents as being in the country illegally. Most had moved to the UK as children in the 1950s and 70s, and found themselves classed as immigration offenders when they reached retirement age.

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.

The 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. Publication was expected last September, but has been delayed by a prolonged period of checking findings with ministers and officials involved in the scandal, to secure their agreement, in a process known as Maxwellisation.

Patrick Vernon, a prominent campaigner for the rights of those affected by the scandal, was invited to Downing Street on Wednesday night to discuss the report with senior officials, but his meeting was cancelled at the last minute. He was concerned that the report’s findings could be buried during the national emergency.

His concerns were echoed by those whose lives were shattered by the Home Office. 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 back burner because of everything else that’s going on.”

One of those involved in researching the report confirmed the imminent publication, adding: “But, sadly, who is going to be very interested?”

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 had been paid out, shared between 36 people.