HMRC has been clawing back tax on savings interest that either does not exist or is already shielded in an ISA, according to financial advisers who say some savers are being hit with bills based on figures that do not match their own records. In one case, the tax office estimated a saver’s untaxed interest at £3,847 when the actual figure was £94.

The mismatch matters because those numbers can feed directly into tax bills or PAYE deductions, leaving taxpayers to pay first and challenge later. Sarah Weston said some taxpayers had received tax calculations and tax coding notices showing savings interest figures they believed were inaccurate, and warned that those errors could lead to unexpected tax bills or PAYE deductions. She said there do appear to be some instances where the figures used by HMRC do not match the taxpayer’s own records.

The complaint lands at a time when interest from savings has become more visible in household finances, and when more people are depending on correct coding to avoid surprise deductions from wages or pensions. The problem flagged by advisers is not just a matter of small rounding mistakes. They say it includes flawed estimates, duplicated interest and interest that is already held in an ISA, meaning it should not be taxed in the first place.

HMRC said it does not want anyone to overpay or underpay tax. It said it updates tax codes based on the most recent data available from financial institutions and continuously improves its processes to allow payments to be corrected. It also said anyone who thinks the information it has is incorrect should let it know straight away so it can put things right.

The criticism is likely to sharpen political pressure because lawmakers are already seizing on the issue as evidence of a tax authority that is moving too aggressively and not accurately enough. Richard Fuller said that at a time when many are struggling with Labour’s tax hikes and the cost of living, HMRC has been taking money it should not and putting people’s finances under further strain. He said ministers should act to bring HMRC into line and stop what he called unnecessary and unfair charges.

Sir Iain Duncan Smith was even blunter, saying HMRC has been a law unto itself for far too long. He said its failings have been huge, that its oversteps into private affairs are enormous, and that the taxpayer has been left picking up the bill. He added that spying on people’s private lives is not what he would call doing a good job.

The central question now is how many taxpayers have been affected and how quickly the wrong figures can be fixed before they turn into deductions or demands for money that was never owed. For the people already receiving notices with the wrong savings income, the answer has already arrived in their bank balances.