Victims of the Windrush scandal continue to face long waits in receiving compensation, a report by the spending watchdog has found.
The National Audit Office (NAO) said the Windrush Compensation Scheme is yet to meet its aim of paying claimants quickly.
NAO head Gareth Davies said the scheme was rolled out before it was ready for claims, causing long waits for victims.
The Home Office said it will work hard to ensure payments are made faster.
The scheme was launched by the Home Office in April 2019 to offer payments to people, mainly from the Caribbean, who came to the UK legally but did not have the documents to prove their right to remain.
In 2018, it emerged that many had lost homes, jobs and access to welfare benefits and NHS services - while some were wrongly detained and even deported.
The payments are intended to compensate victims for things such as loss of earnings, periods of detention and the impact that the scandal had on their lives.
According to the report, when the Home Office started the scheme, it estimated it could pay out between £120m and £310m to 15,000 people based on information from its immigration systems and the 2011 census.
In October 2019, the department reduced the amount it expected to pay to between £60m and £260m to 11,500 people, the NAO said.
But the watchdog found that it takes an average of 154 staff hours to process a case through to payment being approved - which it says is "considerably longer" than the 30 hours the Home Office initially estimated when the scheme began.
Its analysis also found half of cases were "returned to caseworkers for further work" but "some claims have proceeded to payment without errors being identified".
By March this year, the NAO said the Home Office knew of six overpayments, totalling £38,292. The watchdog said it had found further errors and inconsistencies in how caseworkers calculated compensation.
In March, the Home Office "had a budget of £15.8m to run the scheme, and spent £8.1m, of which £6.3m has been spent on staff", the NAO said.
It found the department had "considerably fewer staff than it expected to need" but conceded that claim numbers had been "much lower than expected".
The Home Office originally said it needed 125 full-time caseworkers, it added, but when the scheme launched it had six full-time staff in post.
Last month, the department told the NAO it needed 51 full-time caseworkers and had 53 in post. It is said to be considering hiring 10 more caseworkers before the end of June.
In December, Home Secretary Priti Patel announced the minimum payment awarded to victims would rise from £250 to £10,000, and the maximum from £10,000 to £100,000.
She also said the figure would be higher still in "exceptional" circumstances, with money coming through quicker than before.
Meanwhile, just 19% of those thought to be eligible for compensation have come forward, the NAO found, with 2,163 claims received on Home Office systems by the end of March.
Of these, 1,732 were from individuals, 313 were from family members and 118 claims were from the estates of those who are deceased.
By the same date, £14.3m had been paid to 633 people, but some of these could get further payments, the report said, with £11.6m of this paid since the policy changes it announced in December.
Campaigners have raised concerns about the low number of compensation payments to victims, and lawyers have called the delays unacceptable.
"People are taking a year, 18 months, to even get their first offers," Jacqueline McKenzie, a human rights lawyer, told the BBC. "So it says something is very, very wrong."
Home Office figures released to the BBC's Westminster Hour programme in November last year revealed that at least nine people died before receiving money applied for through the scheme.
Gareth Davies, head of the NAO, said: "The Windrush Compensation Scheme was rolled out before it was ready to receive applications and two years after it was launched people are still facing long waits to receive their final compensation payment."
He said since December last year, the Home Office had "made some progress" but it needed to "sustain its efforts to improve the scheme to ensure it fairly compensates members of the Windrush generation in acknowledgement of the suffering it has caused them".
Yvette Cooper, chairman of the Commons Home Affairs Committee, said it was "completely unacceptable that the vast majority of those affected still haven't received a penny in compensation".
A spokeswoman for the Home Office said it is "determined to put right the terrible injustices faced by the Windrush generation by successive governments" and is in the process of hiring more caseworkers, adding: "We know there is more to do and will continue to work hard to ensure payments are made faster and the awards offered are greater."
The Windrush generation are people who arrived in the UK between 1948 and 1971 from the Caribbean.
They were granted indefinite leave to remain in 1971, but thousands were children who had travelled on their parents' passports - so could not prove they had the right to live in the country.