The Sayce review found between 2015 and the summer of 2025 the official Carer's Allowance guidance was "unclear" and "ill-defined" - preventing many carers from properly reporting their earnings.
Unpaid carers who look after loved ones for at least 35 hours a week can claim £83.30 a week in Carer's Allowance, as long as their weekly earnings stay under £196.
But under a so-called "cliff-edge" earnings rule, anyone who exceeds this limit by as little as 1p must repay that entire week's carer's allowance.
The Department for Work and Pensions (DWP) also levies a £50 penalty on anyone who fails to properly declare a change in their earnings.
The law allows income to be "averaged" over time, but Ms Sayce found "broadly drawn" regulations led to multiple interpretations by both DWP and the courts.
As a result, carers who worked irregular hours or received variable wages often assumed their income could be averaged, but were instead fined by DWP.
In her report Ms Sayce said "this wasn't wilful rule-breaking - it simply wasn't clear what earnings fluctuations carers should report".
The report found the DWP also largely failed to warn carers in time when they went over the earnings limit, meaning overpayments in some cases were allowed to accumulate "for months or years" before unwitting carers were handed huge bills.
In a statement on Tuesday, the DWP said it will begin reassessing cases immediately and potentially cancel or repay debts.
Ms Sayce welcomed the announcements, saying the policy had "major impacts on carers' health, finances and family wellbeing".