Figures showing 371,000 immigrants were on benefits were rushed out by ministers with insufficient regard for "weaknesses" in the data, a watchdog has complained.
In a strongly-worded rebuke to Work and Pensions Secretary Iain Duncan Smith, the head of the UK Statistics Authority condemned the handling of the research.
Despite being "highly vulnerable to misinterpretation", the claims were given to the media without the safeguards demanded for official statistics, Sir Michael Scholar said.
By issuing the figures to the press as a "research paper", the Department for Work and Pensions (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."
Significant media attention was given last week to the figures when they were released by employment minister Chris Grayling and immigration minister Damian Green.
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."
|Keyword||1 POLITICS Welfare|