The Home Office failed to act on repeated warning signs of Windrush failings and is yet to establish the full scale of the scandal, a major new report has found.

According to the National Audit Office (NAO), the department was aware of “credible information” about possible issues as long as four years ago.

In a report published today, Whitehall’s spending watchdog also claims official impact assessments about “hostile environment” measures did not give sufficient consideration to the risk of unfair consequences.

It concludes some Home Office processes contributed to the risk of wrongful detentions and removals.

Head of the NAO, Sir Amyas Morse, said: “The treatment of people who had a legitimate right to remain in the UK raises grave questions about how the Home Office discharged its duty of care towards people who were made vulnerable because of lack of documentation.

“It failed to protect their rights to live, work and access services in the UK, and many have suffered distress and material loss as a result. This was both predictable and forewarned.”

In its evaluation of how the Home Office has responded to the Windrush scandal, the report points out that it has “decided to narrowly focus its historical reviews on individuals from the Caribbean”.

Addressing this, Morse says the department is “taking steps to put things right for the Caribbean community but has shown a surprising lack of urgency to identify other groups that may have been affected”.

Ministers faced a furious backlash over the treatment of the Windrush generation, named after a ship that brought migrants to Britain from the Caribbean in 1948.

Commonwealth citizens who arrived before 1973 were automatically granted indefinite leave to remain but many were not issued with any documents confirming their status.

A public outcry erupted earlier this year after it emerged long-term UK residents were denied access to services, held in detention or removed despite living legally in the country for decades.

The NAO has now said the Home Office “did not act on credible information about issues that may have contributed to the Windrush situation”.

It cites a 2014 report by the Legal Action Group, which flagged up the potential adverse impact of immigration policy on certain groups, including Jamaicans who arrived pre-1973.

In addition, Caribbean ministers raised Windrush cases with the government at a forum in April 2016, while inspection reports highlighted issues including the possibility people were being sanctioned because of incorrect data.

The NAO said: “It is our view that there were warning signs from enough different sources, over a long enough period, to collectively indicate a potential problem that merited further investigation.”

A 2016 recommendation to “cleanse” a database of individuals wrongly flagged as being in the UK illegally was ignored, the watchdog added.

It found the Home Office has not yet established the full extent of the problems affecting people of the Windrush generation.

An official review of 11,800 cases of Caribbean Commonwealth individuals identified 164 people who were removed or detained and might have been resident in the UK before 1973.

The NAO said there are no plans to review around 160,000 files relating to non-Caribbean Commonwealth nationals on the basis this would be “disproportionate”.

“In the circumstances, we find this surprising,” the report said.

It also revealed at least 25 people may have been incorrectly sanctioned under hostile environment policies, such as having a driving licence revoked.

The Home Office has had targets for removing illegal immigrants since 2004, the report noted, but it said there is insufficient information to conclude on whether this contributed to Windrush.

Meg Hillier, chairwoman of the Commons Public Accounts committee, said the impact on those unfairly treated has been “immense”, adding: “It is shocking that the Home Office is not proactively reviewing other Commonwealth nationals’ cases.”

Home Secretary Sajid Javid has issued a profound apology to the Windrush generation and is “absolutely determined to right the wrongs of the past”.

A spokesman for the Home Office said: “As the NAO’s report acknowledges, our taskforce has taken thousands of phone calls and helped over 2,400 people of any nationality prove their status in the UK.

“The majority of those helped by the taskforce are of Caribbean origin, but we have always been clear that it accepts applications under the Windrush scheme from people of any nationality who arrived in the UK before 31 December 1988 and are settled here.

“We have worked hard to raise awareness of the support on offer across a wide range of communities.”

An independent “lessons learned” review has been set up and details of a compensation scheme for those affected will be outlined in the new year, the spokesman added.