A Romanian or Bulgarian family of four would be eight or nine times better off if they were to move to Britain when access restrictions are lifted at the end of the year, immigration campaigners claimed today.
A Bulgarian family - made up of one individual on minimum wage with a dependent spouse and two children - has a weekly income of £62 at home, compared with the £543 they could pocket in the UK, while Romanian families earn around £70 a week.
Elsewhere, single workers from the Eastern European countries would be four or five times better off, the study by Migration Watch UK found.
And a comparison with the conditions Polish workers enjoy in Britain showed that the economic incentives for Romanian and Bulgarian workers were twice as great.
Migration Watch UK chairman Sir Andrew Green said: "The wage differences turn out be simply stunning.
"The previous government made a huge mistake by agreeing to accession treaties that granted full access to our labour market to workers from countries that have only a fifth of our GDP per head.
"Given that the economic incentives for Romanian and Bulgarian workers are twice those now enjoyed by Polish workers, it would be absurd to suggest that there will not be a significant inflow."
Temporary curbs were imposed on Romanians and Bulgarians in 2005 to protect the British labour market, but they expire in December and under EU laws cannot be extended.
The Government has refused to provide an estimate on the number of Romanians or Bulgarians it expects to arrive in Britain, but Migration Watch previously said it could be up to 50,000 a year for the first five years.
A single person in Romania on the minimum wage would have a weekly income of 164 Romanian new lei - about £55 - whereas in the UK that same person could earn £254. In Bulgaria, a single person on minimum wage earns around £49 or 58 Bulgarian lev.
Turning to welfare, Migration Watch said child benefit for two children - even if they remain in their home countries - equals a week's take-home pay at the minimum wage in Romania.
And if the worker should lose their job in Britain, unemployment benefit in the UK is equivalent to more than twice the take-home pay at the minimum wage in Romania or Bulgaria.
Leading politicians from Bulgaria and Romania have dismissed fears that the change in access restrictions will trigger a wave of immigration to the UK.
Bulgarian foreign minister Nikolay Mladenov said many of his compatriots would much rather travel to Germany, Spain and Italy, as his country has stronger business links with those countries.
And Romanian ambassador to the UK Dr Ion Jinga said many immigrants had already bypassed the restrictions by declaring themselves self-employed and finding jobs in sectors such as construction.
A Home Office spokeswoman 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 across government to look at the pull factors that may encourage EU nationals, including those from Bulgaria and Romania, to come to the UK.
"The Government has made clear it will continue work to cut out abuse of free movement."