The government could end up paying as much as £570m in compensation to people affected by the Windrush scandal, but lawyers warned that the design of the scheme meant individual payouts could prove to be very ungenerous.

Because the total number of people affected by the Home Office’s mistakes is not known, the Windrush compensation policy impact assessment provides a range of possible total costs of the scheme, acknowledging that if about 30,000 people apply for compensation, the costs could hit almost £600m.

The department predicts that a more likely number of claims would be about 15,000, in which case the total cost of compensating individuals for losses that arose as a result of difficulties proving that they were not illegal immigrants will be somewhere between £120m and £290m. The home secretary, Sajid Javid, said earlier this week that he expected compensation to be in the region of £200m.

However, there was growing concern from immigration lawyers and campaigners that despite this apparently generous hypothetical sum, and although the government is not introducing an overall cap on compensation payments, there are some very low individual maximum payments for specific losses within the scheme, which effectively equate to a cap on how much individuals will receive.

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The scheme will offer a flat payment of £10,000 for people who were wrongly deported, but just £1,000 for people who were obliged to leave the country under a so-called “voluntary” return scheme because they were unable to prove their right to reside here.

Many people received letters from the Home Office wrongly informing them that they were illegally in the UK, and instructing them to leave if they did not want to be detained. Some felt so harassed by these letters that they agreed to take part in voluntary removal schemes, leaving a country where they had lived for four or five decades; lawyers said £1,000 would not represent fair compensation in those cases.

The immigration lawyer Jacqueline McKenzie, whose firm McKenzie, Beute and Pope is giving pro bono support to a number of people affected, said she welcomed the announcement of the scheme, but said the restrictive tariffs within the scheme made her feel “wary”.

She highlighted the maximum £500 payment for legal costs incurred by individuals trying to resolve their situation, which she said was likely to be much less than many individuals would have paid as they attempted to extract themselves from their Home Office problems. She also pointed to the maximum payment of £500 for people who have been denied the chance to go to university. “That’s insultingly low. Expectations have been raised and some people will get peanuts,” she said.

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For those who lost their job but are unable to provide detailed proof of their employment history there is a maximum compensation payment of £1,147 a month, which is capped at a year’s worth of payments, £13,764, which is about what a minimum wage annual salary would be.

Many of those affected were unable to work for a number of years, so will find this payment very ungenerous. The Home Office said that if individuals were able to show documentary evidence of their exact loss of earnings, these would be paid, but the flat payment was for those who struggled to provide proof.

People who can show that being wrongly classified as an illegal immigrant had a “profound” and likely irreversible impact on their life may be eligible to claim £10,000 or more.

Omar Khan, director of the Runnymede Trust, the race equality thinktank, said: “We are pleased to see the compensation schemes but have concerns that, while it may look generous, it requires applicants to have significant documentation. Given that the Windrush injustice emerged in large part because of documentation requirements being too onerous, there is a significant risk that the scheme will prove less generous to the people affected.”