Almost one in four of those bills, around 317,000 cases, demanded £100 or less. Close to half – 647,000 or 49% – were for £300 or below, piling pressure on households already grappling with rising living costs.

A simple assessment allows HMRC to claw back underpaid tax without requiring the individual to complete a self-assessment return.

They are most commonly sent to pensioners and employees where HMRC believes the calculation is straightforward and it already holds enough income information.

Yet the sheer scale of the operation has raised eyebrows. HMRC has previously defended using private debt collectors to pursue debts as low as £89, arguing it has a legal duty to collect every pound owed to the Exchequer.

The surge has been driven largely by frozen income tax thresholds, introduced in 2022 by the Conservatives and now set to remain in place until 2031 after Chancellor Rachel Reeves extended the freeze by a further three years.

As incomes and pensions edge up, more people are being dragged into the tax net by fiscal drag.

In her November Budget, Ms Reeves sought to ease the pressure on some retirees, pledging that from April 2027 pensioners whose only income is the state pension would not be required to pay “small amounts of tax” through simple assessments.

But campaigners say many others will still be caught out. Steve Webb, a former pensions minister and now a partner at consultancy LCP, said: “Far too many people are receiving demands for trivial amounts of money which in some cases probably cost more to collect and process than they raise in tax.

“In the Budget, the Chancellor said that the Government would not be collecting ‘small’ amounts of money from a select group of pensioners in the coming years, but will apparently go on collecting even smaller amounts from other pensioners and savers through the ‘simple assessment’ process.

“Instead of allowing the system to evolve by stealth, it is time the Government took a step back and assessed whether all the bureaucracy involved in this entire process represents unnecessary hassle for taxpayers and a waste of official time and effort.”

Data shows the number of people being automatically assessed for underpaid tax is accelerating rapidly.

HMRC typically issues a simple assessment where it believes the sums involved are easy to calculate, but experts warn the letters can still come as a nasty shock.

Many recipients assume they fall below the £12,570 tax-free personal allowance, only to discover that a combination of the state pension and modest extra income has tipped them just over the line.

LCP expects the number of simple assessments to exceed two million in 2024–25, when the next set of figures becomes available.

Ian Futcher, of wealth manager Quilter, told the Telegraph: “The steady rise in HMRC asking people for trivial amounts of tax back via simple assessment is a direct result of fiscal drag.

“As tax thresholds remain frozen while incomes, pensions and investment returns creep higher, more people are finding themselves tipping over the tax thresholds by small amounts, increasing the burden both on the individual and HMRC to administer what often amounts to nothing more than £100.

“Simple assessments are increasingly being sent to people who do not consider themselves ‘taxpayers’, often because a rising state pension combined with even modest extra income has pushed them over frozen thresholds. For those affected, the bill can come as a shock.”

HM Treasury defended the broader tax system, with a spokesman saying: “The tax paid by a worker with a low or average income is at a historically low level and Britons benefit from the highest personal allowance among the G7.

“Our commitment to the triple lock means 12 million pensioners will receive an increase of up to £470 a year – worth £1,900 over the Parliament.”