Net migration from the European Union may have been undercounted by tens of thousands, a new report claims.
Pressure group Migration Watch UK said its analysis suggested the index - the difference between the number of people arriving and leaving - could be running at around 220,000 a year. This is nearly 50,000 higher than the official estimated level.
The claim prompted fresh scrutiny of immigration data, which has been at the focus of debate in the run-up to the referendum on Europe.
Controversy has centred on a discrepancy between estimates of long-term migration and allocations of National Insurance (NI) numbers, which are needed by anyone wanting to work.
In the year to September, more than 650,000 NI numbers were registered to EU citizens. Over the same period, immigration figures indicate 257,000 people arrived from the bloc.
NI allocations can include those who leave the UK after a short period or have already been in the country for a long period before applying for a number.
But the large gap between the two sets of figures has fuelled claims that immigration may have been underestimated.
Figures on NI numbers which are actively being used are set to be published ahead of the poll in June after the Government came under pressure on the issue.
Migration Watch, which campaigns for tighter immigration controls, said it compared net migration figures with population estimates for migrants born in the group of Eastern European countries including Poland and the Czech Republic, known as the EU8 nations.
Its report said: "Between 2010 and 2015 the population born in the EU8 and living in the UK increased by an average of 90,000 a year but during the same period estimated net migration from the EU8 averaged only 40,000.
"This suggests that EU8 net migration has been undercounted by 50,000 a year in the last five years."
In the latest official figures, overall international net migration was estimated at 323,000 for the year ending September - more than three times the government's aim of less than 100,000.
Estimated net EU migration to the UK was 172,000 and 191,000 for non-EU migration, while 40,000 more Britons left the country than arrived.
The report said: "Official figures for Eastern European net migration could have been underestimated by more than 50,000 a year in each of the last five years.
"If so, this would mean that net migration from the EU is actually running at about 220,000 year and that the EU is now the largest source of foreign migration to the UK.
"It would also mean that total net migration to the UK is currently running at about 375,000 a year."
Lord Green of Deddington, chairman of Migration Watch, said: "This analysis casts serious doubt on the accuracy of our immigration figures."
The Office for National Statistics said it is working closely with the Department for Work and Pensions, HMRC and the Home Office "to analyse and reconcile all of the various sources of data on migration and short-term workers from overseas". A report of the findings will be published next month.
Matthew Elliott, chief executive of Vote Leave, said: "The public should be told the facts when it comes to immigration and our borders so they can make an informed choice on 23 June.
"The Government does not want to admit EU membership means giving up control and the resulting true level of immigration.
"If we Vote Leave we can take back control of our borders and spend the £350 million we hand to Brussels every week on our priorities like the public services that are under pressure because of the current level of immigration."
A Home Office spokeswoman said:"It is absolutely essential that immigration statistics are robust and reliable, to inform Government policy and be trusted by the public.
"That is why we continue to work closely with the ONS and support its work in providing an accurate understanding of all migration to and from the UK."