The Treasury select committee, which has reported previously on the issues, says the problem has arisen since child benefit was reformed so that for couples where one partner earns between £50,000 and £60,000, a progressively rising tax charge is incurred. At incomes over £60,000, the tax charge wipes out the value of the child benefit entirely.

However, the committee points out that registering for child benefit builds up entitlement to the state pension for parents of children under 12 who do not pay National Insurance contributions, for example because they decide to stay at home to look after their children and do not have paid work. If the parent does not register for child benefit, they may forgo their entitlement to National Insurance credits, and therefore part of their future state pension.

When the tax charge was introduced in January 2013, HMRC wrote to affected households asking if they wished to opt out of receiving the benefit. Those who opt out continue to get the National Insurance credit required for the full state pension.

However, MPs maintain that parents who have started a family since January 2013 may have seen no advantage in registering for child benefit due to the tax charge. They argue these families – for instance, consisting of a higher earner and one stay-at-home parent – could be missing out on the National Insurance credits required for a full state pension.

Since August 2015, each Child Benefit Statistics: Geographical Analysis report produced by HMRC has repeated the phrase: ‘The low number of children at ages two or younger is likely to be because families subject to the high income child benefit charge, whose first child was born since January 2013, would have had a choice between either registering for child benefit and then opting out, or not registering to begin with.

‘These figures suggest that, some of these families may have chosen not to register after learning about the high income child benefit charge.’

However, the Treasury committee says the repetition ‘indicates a concerning lack of attention to the data and analysis on the impact of the charge’, since with each new edition, the age should have risen by one each time: that is, the 2016 edition should have referred to children at ages three or younger, and the 2017 edition four or younger.

Nicky Morgan, chair of the Treasury committee, has written to Mel Stride, financial secretary to the Treasury, to ask for further information, including details of the number of people affected, how HMRC is informing those who may have their pension entitlement affected, and whether the Treasury has undertaken any analysis of the problem.

Morgan said: ‘This problem was wholly foreseeable and preventable. Parents have not registered for Child Benefit for fear of the higher rate tax charge. They may be forgoing their National Insurance credits, and therefore part of their future state pension.

‘The Treasury committee warned that this may happen seven years ago. It urged the government to provide parents with clear information about how their pension entitlement could be affected by the charge. HMRC stated that a clear communications strategy was being developed. It appears that this strategy has not been up to scratch, to the cost of thousands of parents.’

Nicky Morgan’s letter to Mel Stride about child benefit is here.

Report by Pat Sweet