The Institute for Public Policy Research (IPPR) predicts 56,000 Romanian and Bulgarian workers will come to the UK if their countries join the European Union next year.
The think tank, estimated 41,000 Romanians and 15,000 Bulgarians would come and work in the UK next year. The two countries have the opportunity to join the EU from January 1, but a final date has not been set.
The Institute for Public Policy Research (IPPR) urged the Government to give the new arrivals the same full working rights granted to nationals of the eight other Eastern European Countries in 2004.
When eight Eastern and Central European countries joined the EU in 2004, the British government greatly underestimated how many Eastern Europeans would come to the country.
Research suggested the annual figure would be no more than 13,000, but those figures were incorrect when 345,000 signed up to the work registration scheme from May 1, 2004 to the end of 2005.
The IPPR said its 56,000 figure was based on what happened after the 2004 EU expansion.
"Workers from these countries will join an increasingly important eastern European workforce currently doing hard-to-fill jobs in key sectors and regions," IPPR associate director Danny Sriskandarajah.
Chairman of think tank Migrationwatch, which campaigns for tighter rules on immigration, attacked the IPPR's figures.
Migrationwatch's Sir Andrew Green said, "This is a back-of-the-envelope calculation which assumes that immigration from Bulgaria and Romania will be similar to that from the last batch of Eastern European countries."
About 13,000 Romanians and Bulgarians are already in the UK according to the 2001 census.