Experts are warning of a 'significant spike' of newcomers when limitations on coming to Britain, which have been imposed on the citizens of the two countries by the European Union, are lifted at the end of next year.
They believe the numbers will be higher than those seen after Poland and seven other eastern European countries were given the same rights in 2004 because of the fragile state of economies on the continent.
When the open door policy begins in 2014, the door will be wide open to millions from Romania & Bulgaria
Home Secretary Theresa May has indicated support for an end to free movement of EU workers - but Britain cannot stop the restrictions being lifted without tearing up the treaty signed with Bulgaria and Romania when they joined the EU in 2007.
The Government has not made official predictions of how many Bulgarians and Romanians will move here and critics say this is because it grossly underestimated the numbers in 2004.
It predicted less than 20,000 would arrive but Office for National Statistics figures show more than 600,000 were working in Britain last year.
A report by the Coalition's Migration Advisory Committee has highlighted fears that the British jobs market will suffer 'adverse' effects.
Cambridge University emeritus economics professor, Robert Rowthorn, explained around one million eastern Europeans moved to Britain in 2004 and said net migration is around 40,000 a year.
He told The Sunday Telegraph: 'I imagine a similar pattern will be repeated with Romania and Bulgaria, although the transitional controls have perhaps taken the edge off somewhat.'
MigrationWatch UK director, Sir Andrew Green, said Britain has taken far more eastern European migrants than any other country and called for a delay in the lifting of restrictions.
He added: 'I think there could be a significant spike from Romania and Bulgaria, particularly as the economies in other parts of the EU are suffering serious difficulties.
'Neither Spain nor Italy are a good bet at the moment if you’re looking for a job. I think we need a further five year extension of the transitional arrangements.'
Figures show nearly 40,000 Bulgarians and Romanians moved to the UK last year, with more than 130,000 living in Britain - where the GDP per capita is nearly four times higher.
An investigation by The Sunday Telegraph revealed that loopholes allowed 50,000 east Europeans to get jobs they should have been restricted from by declaring themselves 'self-employed'.
These included hotel and restaurant workers, sales staff, taxi-drivers and lap-dancers - with many thousands of such jobs being advertised in the two countries.