Iain Duncan Smith and the Department of Work and Pensions have been accused of publishing misleading immigration figures that were "highly vulnerable to misinterpretation".
Figures showing 371,000 immigrants were on benefits were rushed out by ministers with insufficient regard for "weaknesses" in the data, according to the UK Statistics Authority on Wednesday.
In a strongly-worded rebuke to Work and Pensions Secretary Iain Duncan Smith, the head of the UKSA, sir Michael Scholar, condemned the handling of the research, the Press Association reported.
Sir Michael said that despite being "highly vulnerable to misinterpretation", the claims were given to the media without the safeguards demanded for official statistics and by issuing the figures as a "research paper", the DWP had bypassed the need to meet the usual code of conduct, he noted.
Sir Michael, who has led campaigns against Government spinning of official statistics, said the move had also allowed ministers to issue a "political commentary".
But he said it was hard to see how that position could be justified and called for assurances that any future release would be treated more carefully.
The watchdog is frustrated that it has no way to challenge the decisions of Government departments on how information is classified and wants extra powers to do so.
A spokesman said: "The Authority would wish to be the arbiter on what is and what is not an official statistic given the Authority's other statutory responsibilities."
The number was arrived at by cross-checking welfare, border and tax records for the first time to establish the nationality of people when they first claimed benefits.
All but 2% were found to have genuine claims, with a majority having been granted UK citizenship since arriving in Britain.
In a letter to Mr Duncan Smith, Sir Michael noted that the DWP's own website listed the research as statistics - and that it was based entirely on existing official information.
"These statistics are both highly relevant to public policy and highly vulnerable to misinterpretation," he said.
"There are some important caveats and weaknesses that need to be explained carefully and objectively to Parliament and the news media at the time of publication.
"This is, in our view, best done by official statisticians producing a statistical release in accordance with the Code of Practice.
"With these considerations in mind, I seek your agreement that any further publication of these, or of any such statistics, be handled as an official statistics release."
Labour said the ministers had been caught "misusing statistics for their own political ends" and a trade union accused the Government of deliberately trying to "whip up suspicion".
Shadow immigration minister Chris Bryant said: "Once again, the Immigration Minister and the Employment Minister have been found to be misusing statistics for their own political ends.
"And this is not the first time Sir Michael has had to reprimand the ministers in question - both have a track record of twisting official statistics to mislead the public for their political gain."
A spokesman for the Public and Commercial Services union, whose members work in benefit offices and jobcentres, said: "The release of misleading and highly-politicised information from the DWP press office is fast developing into a worrying trend... It's a disgraceful way for a Government to behave."
Mr Duncan Smith said he would consult with the watchdog over how such information would be published in future but that there were "no plans at present to repeat this analysis".
In a reply to Sir Michael, Mr Duncan Smith insisted that the figures were prepared "to appropriate professional standards".
The DWP had made available "all relevant information about the sources of the data and any caveats or limitations" and ministers had taken professional advice throughout as to the status of the release from the senior DWP statistician, who would discuss the case with UKSA.
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