The Cabinet secretary has apologised after Tony Blair broke protocol by pre-empting the release of official unemployment figures in a speech.
Unemployment has risen over the past year, despite economic growth
Sir Gus O'Donnell expressed "regret" in a letter to the head of the Office for National Statistics (ONS).
The row between ONS and the government comes as ILO unemployment rose by 93,000 between May and July to 1.7 million, its highest level since 2000.
Unemployment benefit claimants in August fell by 3,900 to 950,100.
The unemployment rate was 5.5% in the quarter, up 0.3% on the previous three months.
Average earnings rose 4.4% in the year to July, up from 4.3% in June.
The latest figures come a day after Tony Blair told the TUC Conference in Brighton that they should show a "very welcome" fall in unemployment.
While it was the number of people claiming unemployment benefit that actually fell, this figure was out of line with previous trends chief economist at the Chartered Institute of Personnel and Development, Dr John Philpott said.
"The significance should not be overstated. It's likely to be subject to revision," he added.
Both the Conservatives and Liberal Democrats accused the prime minister of abusing his position and trailing official figures in advance of their release.
National statistician and Registrar General Karen Dunnell wrote to Sir Gus on Wednesday, saying the Prime Minister's comments "clearly contravened" the National Statistics Code of Practice.
"It is essential for the maintenance of the integrity of official statistics, and of public confidence in them, that the provisions of the code of practice are fully complied with," she said.
The data had been released to Mr Blair's office at 5pm on Monday but with usual restrictions on when they could be released.
Sir Gus said that Number 10 Downing Street was "fully seized" of the importance of complying with the rules.
Mr Blair made his comments on unemployment in a speech to delegates at the TUC conference on Tuesday.
He said that "tomorrow I think we will probably see - for the first time in some months - a fall again in unemployment, which is very, very welcome indeed".
The Tories accused Mr Blair of making an "irresponsible gaffe".
"Important economic data should not be blurted out at meetings with trade unionists," said shadow chancellor George Osborne.
"This proves why we need independent statistics that are not subject to ministerial spin.
"Last year Gordon Brown promised that would happen. But he has failed to deliver," Mr Osborne said.
Analysts said that the unemployment figures presented a mixed picture that was unlikely to stop the Bank of England raising interest rates again this year.
The Bank is widely tipped to raise its borrowing costs by a quarter of a percentage point to 5% in November.
They added that the unemployment figures may well improve in coming months should the economy continue to expand.