BRITAIN’S National Health Service often seems to be stumbling from crisis to crisis, but its latest problem is a self-inflicted wound. Waiting lists for operations in England have risen sharply, with one trust reporting a 50% jump in the past three months. The problem is not caused by a shortage of doctors but rather by the fact that existing staff are refusing to do extra shifts, because of the tax bills they would face. In some cases, the rules mean doctors would in effect lose money by working.

The problem stems from two contradictory policy impulses. On the one hand, the government wants people to save towards a pension, so it gives them tax incentives to do so. But it does not want the hit to its tax revenues to be too great, so it seeks to limit the size of the pensions break. High earners (like hospital consultants) pay a marginal tax rate of 40% or 45% on their income and have a particular incentive to avoid tax by making a pensions contribution. In the tax year 2010-11, such earners could contribute £255,000 ($320,000) into a pension without paying tax.

This was expensive, so David Cameron’s government drastically limited the annual amount people could contribute to a pension, free of tax—first to £50,000, and then to £40,000 from 2014-15 onwards. But the real complications came with a change to the rules in 2016-17, which introduced a “tapering” of the annual allowance.

Tapering is based on two key levels of income. The first is the “threshold income” rate, of £110,000. This is based on the worker’s salary. But once the threshold is passed, employees can fall foul of a second number, the “adjusted income” level, of £150,000. The “adjusted income” figure includes any increase in a worker’s pension rights, and not just their pay. Contributions made by the employer count towards this amount. The resulting “pension growth” figure is calculated on the basis of a complex formula, but is usually a multiple of many times any extra pay the employee has received.

The British Medical Association (BMA) gives a real-life example of a consultant who agreed to take on an extra management role in the face of staff shortages. This extra responsibility came with a reward of £10,000 a year. Before the increase, the doctor was earning £101,000. The pay increase took his salary above the threshold income level of £110,000. And the pay increase was then converted into a “pension growth” figure of £107,000, taking him well above the adjusted income level as well. The result was a one-off tax bill of £42,000, or more than four times the pay increase. Because of his extra work, the consultant was worse off.

The problem has taken a while to kick in because high earners were allowed a three-year period when they could carry forward tax allowances from previous years. That time is now up and so the full weight of the post-2016 rules applies.

Doctors are balking at covering for their colleagues, for fear of the tax burden that may result. What makes life more difficult is that they cannot be sure, over the course of the year, how much income they will earn and thus whether the thresholds will be triggered. The rules are also complicated by the existence of two different NHS pension schemes, one based on a doctor’s final salary and another on the average earnings over their career.

These complexities will be tricky to solve in the short term. Any changes made by the chancellor in his next budget are likely to apply only to subsequent tax years. Meanwhile, the BMA says half of all consultants plan to retire by the time they reach 60, and half of those cite pensions taxation as an important factor in their decision.