Just 4,200 Brits have been handed welfare back payments in a massive review the Tories claimed could benefit 220,000.
Ministers agreed in 2018 to pay vast numbers of people with mental health issues more Personal Independence Payments (PIP) after a court defeat.
Fearing the change would cost £3.7bn over five years, ministers initially rewrote the law to avoid obeying the tribunal.
But after an outcry, they U-turned and agreed payments for those affected.
Estimates at the time suggested 164,000 people could benefit, and then-welfare chief Esther McVey later said “it could affect up to 220,000 people”.
But it’s understood the initial estimate was based on a sample of just 60 people, and later estimates were of how many people “might” benefit.
Department for Work and Pensions (DWP) officials now admit the real figure is much lower.
Sophie Corlett of mental health charity Mind said: "Right now, many people with mental health problems in receipt of benefits are already really struggling financially, sometimes having to skimp on heating or food.
"The courts have already said people should be in receipt of their full PIP, so people should be urgently given their backdated payments. This is even more important as the cost-of-living rate soars.
"Thousands of people are being left without the financial support they need without reason or apology – pushing more into real hardship - it is an appalling state of affairs."
The decision brought the government in line with a tribunal judgement in November 2016.
Known as the ‘MH’ case, it ruled people who suffer "overwhelming psychological distress" when travelling alone should qualify more easily for PIP.
Almost a million cases were reviewed against the ‘MH’ tribunal as of November 1, but only 4,200 received arrears payments totalling £22m.
A further 4,100 people received another £22m of arrears connected to a different tribunal judgement known as ‘RJ’.
DWP is planning to write to another 180,000 claimants who fought a tribunal themselves, inviting them to have their claim reviewed.
But these letters will only be sent in 2022 - six years after the original ruling.
Anyone eligible for a payment receives it backdated to the original tribunal.
A Department for Work and Pensions spokesperson said: “We have paid £22m in back payments to thousands of claimants, and made it clear our early estimates of those affected were likely to change.”
