The two countries are already members of the European Union. In 2014, their citizens will be able to move freely around Europe when transitional controls are lifted.
A Cameron spokeswoman said the prime minister opposes the "abuse of free movement," The Guardian reported.
"What we are, and have been, very clear about is we welcome the brightest and best to the U.K. and support immigration that helps the U.K. labor market and our economy," she said.
Liberal Democrats, junior partners with the Conservatives in Cameron's coalition, warned that Britain cannot discriminate against specific countries.
"We are up for making sure that the rules are correct," a Liberal Democrat told the Guardian. "But they should not just apply to Romanians and Bulgarians. We have to look at the rules for all EU migrants."
Since Poland and seven other Eastern European countries joined the EU in 2004 with no controls on free movement, hundreds of thousands of immigrants have settled in Britain. The Census reported earlier this week that Polish speakers are now the biggest foreign language group in England and Wales.