The true scale of immigration from Europe is finally set to be revealed amid growing speculation that the official figures are a vast underestimate. With Britain’s referendum on membership of the European Union (EU) set to take place weeks after the figures are released, a drastic revision upwards in migration statistics could spell victory for the Leave campaign.

Concerns have been repeatedly raised as to the accuracy of official migration statistics, as the number of National Insurance numbers handed out over the last four years vastly exceeds the official estimate on how many European citizens have settled within Britain during the same period.

Breitbart London has been at the forefront of raising those concerns, publishing analysis last August which showed that, in 2014/15, some 214,000 National Insurance numbers were handed out to Romanians and Bulgarians, yet just 53,000 migrants from those countries were registered in official statistics.

Jonathan Portes, principle fellow at the The National Institute of Economic and Social Research used the figures as the basis of a Freedom of Information request asking the government to verify how many of the National Insurance numbers were actually in use, therefore giving a true picture of the scale of immigration into the country.

Astonishingly, civil servants refused to release the data – “Not because they don’t have it” Portes reported. “Not even because it would be too much work.”

Rather, they refused to disclose the true scale of immigration as they argued it would interfere with the Prime Minister David Cameron’s renegotiation of some aspects of Britain’s membership deal with the EU.

UK Independence Party leader Nigel Farage picked up on the events in his Breitbart London column last December commenting: “It is bad enough that this government is now running net migration at over a third of a million every year. It is even more shameful that they seem to be covering up what could actually be the real, far higher numbers.

He called on the government to release the true figures, saying: “The government must now release the full data they have for numbers of EU migrants they have coming to the UK rather than attempting to hide their appalling record just so it’s easier for Mr. Cameron to appease Mrs. Merkel in Brussels.

“They have allowed EU migration to run totally out of control. They should be made to stand by their shameful record rather than be allowed to sweep the true figures under the carpet.”

It seems that some within Mr Cameron’s Conservative Party agree. With latest figures from the Office of National Statistics showing 919,000 EU migrants settling in the UK between 2010 and 2014, while 2.25 million National Insurance numbers were issued during the same period, senior Tories have called for the true figures to be released.

“We can know the truth of the matter only if Her Majesty’s Revenue and Customs releases its data on active EU national insurance numbers, but HMRC has refused to do so,” David Davis MP told colleagues in the Commons.

He added: “Will the prime minister instruct HMRC to release those statistics immediately so that we can understand the truth about European Union immigration?”

Last night, following instruction from the UK Statistics Authority that the public must be given the full picture on migration before the referendum, the government agreed to do just that.

A No 10 spokesman said: “The prime minister told the Commons last month that he would continue to ensure HMRC provide greater information on national insurance numbers. HMRC wrote to the Treasury select committee last week to confirm it will be working to provide the Office for National Statistics with additional data.”

Lin Homer, the head of HMRC, has confirmed that the information will be conveyed in a letter to the Treasury Committee by the end of this month, the Guardian has reported. She said that the delay had come about because of the need to combine data sets.

The confirmation has been welcomed by politicians and activists alike who have been pushing for a clearer picture on migration.

Lord Green of Deddington, the chairman of Migrationwatch, said: “This is progress. The Government have at last recognised that they cannot continue to obfuscate on a matter of such importance to the public and possibly to the referendum.”

The information could prove decisive as Britain heads to the polls to vote on whether to remain part of the European Union. The most recent poll of polls puts the result on a knife edge, with the Remain campaign just marginally in the lead at 51 percent to the Leave campaign’s 49 percent, discounting the approximately one in five who are still undecided.