Around 8,000 immigrants a year from Bulgaria could come to the UK when restrictions on their movement in the EU are lifted next year, the country's ambassador has said.
Konstantin Dimitrov told Channel 5 News Bulgarians could not legally be denied access to labour markets once the controls are lifted in January.
Asked about how many the UK could expect, he said: "Maybe 8,000 immigrants a year judging on tendency for this year. While in the Europe Union you cannot stop the free movement of people and their legal access to the UK labour market."
The Government has refused to say how many migrants it expects from Bulgaria and Romania, whose nationals will also have full freedom of movement from the new year.
However the MigrationWatch UK campaign group has estimated that Britain could face an annual influx of 50,000 migrants from the two countries over the next five years, putting severe pressure on housing and public services.
Mr Dimitrov denied that his government was encouraging people to seek work abroad, saying his country was hurt by the "brain drain" of qualified doctors and nurses and financiers and entrepreneurs.
"If you are member of EU there is a competition of all job offers at stake. It's a matter of skills, motivation of people and everyone should compete for jobs offers on an equal basis if every applicant is a citizen of a member of the EU," he said .
Earlier Downing Street confirmed that it is looking at a possible extension of the length of time new arrivals from the EU have to stay in the UK before claiming benefits.
But the Prime Minister's official spokesman poured cold water on reports that David Cameron is ready to defy EU rules in order to impose tougher conditions, telling reporters: "The Government acts within the law."
Asked about reports that the Prime Minister is considering extending the residence period required to gain access to benefits from three months to a year, M r Cameron's official spokesman told a regular Westminster media briefing: "You would expect the Government to be looking at whether there is more that could be done. That process is, as you would expect, going on."
The spokesman said he was not in a position to say when a decision might be made or what changes might be introduced, telling reporters: "I can't speculate about possible timings or measures."
Any extension of the bar on benefits would potentially set up a clash with Brussels and a row within the coalition Government.
The European Commission prevents member states from discriminating between their own citizens and those from other European Union members.
Liberal Democrat Scottish Secretary Alistair Carmichael has warned any change in the UK's approach should be discussed in Europe.