David Cameron insisted today that it was currently impossible to predict immigration levels from Bulgaria and Romania when access restrictions are lifted at the end of this year.
The Prime Minister backed Communities Secretary Eric Pickles, who said yesterday that nobody knew how many people would come from those two countries.
But Mr Cameron said there was a "big difference" between what happened when Poland acceded - and immigration was many times higher than official projections - and the situation with Bulgaria and Romania.
"I completely support what Eric Pickles has said, which is 'Don't make official predictions unless and until you've got real confidence in those figures'," he told the BBC Radio 4 Today programme.
The Prime Minister said it was "a very difficult calculation to make" and "the detail is not there yet".
He stressed that transitional controls were in place this time, unlike when Poland acceded, but added that the Government did not want to give misleading figures like the last Labour administration.
"We don't want to make that same mistake again so it's important to take the time and get this right," he said.
Mr Pickles admitted that any influx from the European Union states would "cause problems" with services such as housing, with the highest numbers likely to arrive in London boroughs which already have significant populations from the countries.
However, he insisted it was not "reasonable" to assume that 300,000 would move to the UK - the figure suggested by some Tories based on migration levels after Poland, the Czech Republic, Latvia and Lithuania acceded.
The comments came after the Coalition confirmed it would not seek to extend temporary curbs on 29 million Romanian and Bulgarian nationals' right to live and work in Britain, which are due to expire in December.
Speaking on the BBC's Sunday Politics programme, Mr Pickles said there had not been any discussions with the Home Office about potential numbers.
He suggested his own department had come up with figures - but refused to say what they were.
"The truth is I don't think anybody entirely knows the number that are going to come from Bulgaria or from Romania," he said.
"I've seen figures, I wasn't confident with those figures. I've asked for a further explanation and when I've got that explanation and when I feel confident about the figures then I'll talk about the figures."
Mr Pickles went on: "Given that we've got a housing shortage, any influx from Romania and Bulgaria is going to cause problems and it's going to cause problems not just in terms of the housing market, but also on social housing markets.
"But one of the reasons why I'm not prepared to start a scare story going is that I think we need to be reasonably confident about the figures."
Labour indicated last year that it supported extending restrictions on migrants from the two countries working in the UK.