The number of EU migrants working in the UK is to be announced before the referendum takes place, which is likely to show the level of immigration has been underestimated by current statistics.
HMRC is to reveal data on how many national insurance numbers are in use by people from the European Economic Area.
A spokesperson from HMRC said: "We’re working hard to pull all the information together and once this is done we will be providing this to the ONS."
HMRC is to release the figures to the ONS, after having been asked by the Treasury Select Committee for the data last year.
"As this letter makes clear, I have now received a firm commitment from HMRC, working with ONS, to provide this information. This has been obtained as a result of a good deal of persistence," said Andrew Tyrie, chair of the Treasury Select Committee.
"Since receiving it, I have pressed further, receiving an assurance that it should be possible to supply the information to the Committee before the end of this month. Late, but a good deal better than never. I recognise that HMRC may have encountered some difficulties. So I am glad that they have found a way of resolving them," he added.
Official statistics count the number of foreigners into British airports, but that figure is different to those given national insurance numbers.
Since 2010 some 904,000 EU nationals moved to the UK, according to figures from the ONS. However, 2.25m national insurance numbers have been issued.
The result is that it's expected that the figures could be higher than statistics currently in the public domain. If true, pro-Leave campaigners are likely to jump on the findings to bolster their arguments of why the UK should exit the EU.
Pro-Leave Tory MP Jacob Rees-Mogg told City A.M.: "The attempt to cover up these numbers is an affront to the constitution. Parliament has a right to such information and this type of delay and obfuscation makes people wonder what the government has to hide."
The data was initially requested by Jonathan Portes, a research fellow at the National Institute of Economic and Social Research. He previously wrote that official statistics were likely to have underestimated immigration, but in the absence of the data said he could not be sure.
"I am delighted that the government has finally agreed to publish some of the data it holds on the number of national insurance numbers issues to migrants that are in use. This will be useful information both for researchers and the general public," Portes said.
He added: "However, it is important that what is published is as comprehensive as possible, and that researchers are able to ask follow up questions. It is also important that the Office for National Statistics have full access to the underlying data, so that they are able to cross-check it against the official immigration statistics."