Last year approaching half a million women opted out of receiving child benefit. An unknown number never applied in the first place. Why? Almost certainly because their partner was earning more than £60,000 a year, which is the final cut-off point for the payment. But while they save their partner the hassle of having to fill in a self-assessment tax form, the ones who have not applied may also be unwittingly denying themselves a full state pension.

This week Nicky Morgan, chair of the Treasury committee, warned that the “link between national insurance credits [NICs] and child benefit is poorly understood by the public,” and that “stay-at-home parents risk losing out on pensions”.

So what’s going on? Back in 2010, the then chancellor George Osborne announced that the government would remove child benefit from households with a higher earner. For couples where one partner earns between £50,000 and £60,000, it is progressively removed, and stops entirely if one earner has an income over £60,000.

As a “tough but fair” austerity measure, saving around £1bn a year, it had fairly widespread support. Why, with the government’s finances in chaos, should the well-off receive benefits worth, at the time, just over £1,750 a year for two children?

But as Morgan points out, it has had an unintended consequence. Under the current system, if either parent or guardian earns more than £50,000, they become liable for the “high income child benefit tax charge”. They can still claim it, but they then have to register for self-assessment and fill in a tax return, and the state claws the child benefit back that way.

Many couples will regard this as putting the money in one pocket then having it removed from the other, so don’t bother to register for child benefit.

But here’s the rub; claiming child benefit, whether it is actually paid or not, ensures the claiming parent receives NICs while the child is under 12. But a mother (and it’s usually the mother) who does not claim, may fail to build up their full state pension entitlement.

To receive a state pension the parent needs to have paid NICs, or received NICs, for a full 35 tax years. The credits are crucial for stay-at-home parents, as they plug the gap in the NICs record while looking after their children.

HMRC figures for 2017 show that 433,665 women opted out of receiving child benefit, but have remained in the NI system and will receive the credits. What we don't know is how many never register.

To be fair, the government warns about the risk. Its website states “you can choose not to get child benefit payments, but you should fill in the child benefit claim form. This will help you get NICs which count towards your state pension.”

At this stage, you might be thinking “poor rich people”. So what if they miss out on a few years’ state pension entitlement, they're loaded. Many probably are. But there are plenty who lose a high-pay job and never recover. So if you are thinking of not applying, think again.

Take the benefit, and regard it as an interest-free loan you'll pay back at the end of the year – and ensure you stay in the NICs system.