Downing Street has withheld publication of a cross-governmental report that suggests one potential impact of immigration is smaller than claimed.
It suggests "displacement" - the number of UK workers unemployed as a consequence of immigration - is well below the figure used by ministers of 23 for every 100 additional immigrants.
This was considered potentially incendiary, BBC Newsnight has learned.
No 10 sources said the report was unfinished and not ready for release.
Home Secretary Theresa May has previously used research from 2012 by the Migration Advisory Committee (MAC), a group of independent academics, as a basis for saying that "for every additional 100 immigrants… 23 British workers would not be employed".
But the new analysis - by civil servants - is much less pessimistic.
The work, an extension of that MAC research, estimates that the cost to existing British workers of new arrivals is much lower.
However, the figure it suggests for the displacement of British workers has not been disclosed.
Officials say the prime minister's team has prevented publication of the report, which has been ready since last year, to avoid igniting controversy.
But Downing Street sources told the BBC the report had not yet been completed and was not ready to be released.
They insisted it would be published "in due course", and the BBC's chief political correspondent Norman Smith says he understands this could be within days.
Statistics released last week suggest the government is unlikely to meet its pledge to reduce net immigration to below 100,000 people a year by 2015.
If ministers wish to get tougher on new arrivals, research finding that immigration is easier to absorb than previously thought is politically unhelpful.
Immigration is also a coalition fault-line. Mrs May, a Conservative, has faced opposition to her plans to cut immigration from the Liberal Democrats.
Some Conservatives have caused problems for her, too. David Willetts, the universities minister, has sought to prevent restrictions on foreign university student numbers.
Home Office officials protest that the new research, in parts, reflects an "institutional bias" in favour of migration among officials at the Treasury, Foreign Office and business department.
According to emails seen by Newsnight, the old research on displacement cited by Mrs May was not considered sufficiently "robust" by either the Treasury or the business department.
The new report, however, has been checked by external academics and agreed across all the departments with a hand in migration - including the Home Office.
Liberal Democrat MP Julian Huppert told Newsnight "it's obvious that this report should be published as quickly as possible".
"We have to have the right figures, so we can make the right decisions, so that we get the best people here to help our economy," he added.
Labour's shadow immigration minister David Hanson agreed the report had to be published, adding that "the British people should have information made available to them so they can make a judgement about the impact of immigration on jobs".
"This should be done on the basis of fact, not more empty rhetoric or spin from the government," he said in a statement.
Conservative MP Stephen Barclay, however, said "the idea that the government is not being transparent when they themselves have commissioned this report is a nonsense".
He also said it was not true that the impact of immigration had been minimal, adding that areas such as his constituency in north-east Cambridgeshire had felt a "big impact on public services, on housing, on wage deflation".
Downing Street said much of the report supported the government's case for tighter controls on immigration.
No 10 said the initial figures, compiled by teams at the Home Office and Business Department, had now been peer reviewed by academics, but that work had still to be completed.
The BBC's Norman Smith says he understands the research has not been seen by the prime minister.