The deputy leader of the Labour party, Tom Watson, has accused the government of hiding essential information before the planned EU referendum by failing to disclose official figures on the number of migrant benefit claimants in Britain.

HM Revenue & Customs is refusing to disclose how many British nationals claiming tax credits are being counted as migrants. The number in question inflates the figure for immigrant families claiming in-work benefits and potentially means any policy aimed at restricting the benefits of EU migrants could hit thousands of Britons.

As the Guardian revealed last October, HMRC defines “non-UK families” as those where at least one adult in the claimant family is a migrant, meaning that mixed families where one partner is a British national are classed as immigrants.

Following that story, a freedom of information request was submitted to HMRC asking how many claimants classed as part of migrant families were British nationals.

Under FoI terms, a response was due by mid-November but the figures have yet to be released.

Although it claims it is dealing with the FoI request, HMRC has refused to say when it intends to respond. The tax office has also failed to explain why it missed the statutory deadline of 13 November or indicate any exemptions it may be considering, which should be communicated in delayed cases.

Watson said: “The fact the government has failed to respond to repeated freedom of information requests to explain how it defines an ‘immigrant family’ suggests it has something to hide. We can’t debate the UK’s place in Europe ahead of an historic EU referendum without accurate statistics on this and other issues.”

“The Freedom of Information Act was introduced by a Labour government because the public has a right to know about the decisions taken in its name. Labour would strengthen the act, but the Tories want to weaken it.”

HMRC’s definition of migrant families not only inflates the figure of 740,000 non-UK families claiming tax credits but also means that any policy aimed at restricting the benefits of migrants could also hit Britons.

More than 7% of the UK’s 15.6m couples comprise one UK national and one non-UK national, according to analysis compiled by the Office for National Statistics for the Guardian. But when any such couples claim tax credits, they could be considered migrant families by the British government.

According to HMRC data there were 738,900 non-UK families (which include single people and couples) in receipt of tax credits as of March 2014, the most recent data released by the tax agency that includes a UK/non-UK breakdown. That is 15.9% of the total caseload.



Of the 738,900 non-UK families, 431,500 are couples while the other 307,400 are single claimants.

However, only 401,700 of all the 2.6 million singles (UK and non-UK) claiming tax credits have no children and receive working tax credit only. HMRC has also refused to say if it knows the number of cases where non-UK single claimants are claiming child tax credit and the other parent is a British national.

The vast majority of tax credit expenditure relates to families with children.

HMRC estimates that annual entitlements of families containing a non-UK national were £5.2bn in 2013-14 (17.4% of the total £29.7bn spend for that period). Of the £5.2bn, £1.2bn was paid to out-of-work families on child tax credits, £3.8bn to in-work families with children, and £200m to in-work claimants without children.

Last December, HMRC refused to disclose how many national insurance numbers issued to recent migrants were “active” (ie showed recent payments of tax or benefit claims) following an FoI request; the tax agency claimed releasing the information would be unhelpful to the UK’s EU membership negotiation process.

An HMRC spokesperson said: “We take our responsibilities under the FoI Act very seriously, and we make every effort to meet the statutory deadline for all FoI requests. Unfortunately, we are sometimes unable to provide a response by the deadline.”

Update: this article was updated on 8 January with a comment from HMRC.