Up to 250,000 Romanian and Bulgarian immigrants could flood into Britain within the first five years after access restrictions are lifted, campaigners warn.
The figure - equal to the population of Newcastle - comes after the coalition Government refused to give an estimate of the numbers of people who might move to Britain after gaining the right to live and work in the UK from the end of December.
Migration Watch UK, which reached the estimate by assessing three different methods of predicting the likely inflow from Romania and Bulgaria, said immigration from the two countries could amount to 50,000 a year between 2014 and 2018.
Temporary curbs were imposed on Romanians and Bulgarians in 2005 to protect the British labour market, but those curbs expire in December and under EU laws cannot be extended.
Migration Watch chairman Sir Andrew Green said: "It is not good enough to duck making an estimate of immigration from Romania and Bulgaria. It is likely to be on a scale that will have significant consequences for housing and public services."
The estimate could be considerably higher if a further one million Romanians in both Spain and Italy decided to transfer to Britain, the campaigners added.
Income per head in Bulgaria and Romania is about one fifth of that in the UK, similar to Poland in 2004, Migration Watch said, while youth unemployment is also high.
With youth unemployment in Spain at 56%, Italy at 37% and France at 27%, the campaign group said it expects only Germany and the Netherlands are likely destinations other than Britain.
A Home Office spokesman said: "We are working to cut net migration from hundreds of thousands to tens of thousands by the end of this Parliament and our tough new rules are already taking effect with overall net migration falling by a quarter in the past year.
"In terms of European immigration, we are working closely with other government departments to look at the pull factors that may encourage EU nationals, including those from Bulgaria and Romania, to come to the UK."