Around 8,000 migrants a year from Bulgaria could come to the UK when EU restrictions on their movement end, the country's ambassador has said.
Konstantin Dimitrov told Channel 5 News Bulgarians could not legally be denied access to labour markets once controls were lifted in January.
The UK government has not revealed how many migrants it expects to come.
Pressure group MigrationWatch UK expects 50,000 people to come from the two countries each year for five years.
Restrictions on migration from Bulgaria and Romania are set to end in 2014.
Asked about how many Bulgarian migrants the UK could expect, Mr Dimitrov said: "Maybe 8,000 immigrants a year judging on tendency for this year."
The ambassador denied his government was encouraging people to seek work abroad, saying his country was hurt by the "brain drain" of qualified doctors and nurses, financiers and entrepreneurs.
'Within the law'
He also rejected the idea they were "stealing" the jobs of people in the countries which they migrated to.
"If you are member of the 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 it was considering extending the time that EU migrants needed to have spent in the UK before they could qualify to receive benefits.
It came after reports Prime Minister David Cameron wanted to quadruple the period from three months to a year.
Mr Cameron's official spokesman said the government was looking at whether "more could be done" but he said he could not "speculate about possible timings or measures".
He also dismissed reports the prime minister was ready to defy EU rules - which prevent member states from discriminating between their own citizens and those from other EU members - in order to impose tougher conditions.
"The government acts within the law," he said.
On Sunday, Education Secretary Michael Gove said Mr Cameron had "struck exactly the right note on migration".
Lib Dem MP and Scottish Secretary Alistair Carmichael warned any change in the UK's approach should be discussed in Europe.
A recent European Commission study found that jobless EU migrants made up a very small share of those claiming social benefits in EU member states.
In most of the EU countries studied the portion of EU migrants among welfare beneficiaries was below 5%.