The criticism came as Britain and Ireland announced a shift from the open-door policy adopted towards other eastern Europeans.
The controls, announced by both countries on Tuesday, respond to growing public concern that EU enlargement is resulting in increased immigration, undercutting resident workers.
Calin Tariceanu, Romania's prime minister, said Tony Blair, Britain's prime minister, had caved in to demands from the opposition.
"Unfortunately, the British government gave in to pressure," he said, promising to fight to remove the controls.
Low-skilled workers from Bulgaria and Romania will be allowed to work in Britain in food-processing industries and agriculture under a quota system.
However, skilled migrants will be granted jobs only if they cannot be filled by residents.
Other states to follow
Britain's restrictions will be reviewed annually. Similar rules are expected to be applied by other western EU states, though Slovakia has announced an open-door policy.
Ireland said Bulgarians and Romanians would have to apply for work permits, with preference over non-EU members.
Critics called the British restrictions unworkable, especially since they exempt self-employed workers, and said they would encourage illegal labour and tax evasion.
"Unfortunately, the British government gave in to pressure"
Romanian prime minister
John Reid, Britain’s interior minister, said in a statement: "The UK will maintain controls on Romania and Bulgaria's access to jobs for a transitional period.
"We look forward to welcoming Romanian and Bulgarian workers here, provided that they comply with our rules and obey the law."
The move comes after London grossly underestimated the numbers of eastern European workers coming to Britain following the EU's enlargement.
In response, Bulgaria said it would consider placing similar restrictions on UK workers.
Dimitar Tsanchev, a foreign ministry spokeswoman, said: "The possibility of reciprocal measures from the Bulgarian side will be discussed as concerns Britain or any other EU member states who impose restrictions for the free movement of workers."
Students from both countries will be able to take part-time work, provided they are enrolled in an approved college.
Britain won favour with the 2004 newcomers when it offered their workers unfettered access. Sweden and Ireland were the only other two EU countries that adopted the open-door policy.
But the government said in August that more than 400,000 workers from the eight former communist states had come to Britain - far higher than the official forecast that predicted the entry of between 5,000 and 13,000 new immigrants.
As part of the changes, London will phase out all low-skilled migration schemes for non-EU workers from January.
The rules will not affect workers from the countries that joined the bloc in 2004.