Mr Campbell has said his diaries are not for publication

Geoff Mulgan, until 2004 a top aide to Tony Blair, told the BBC diary-keeping by insiders damaged trust.

And the knowledge that ex-media chief Mr Campbell and others kept them probably "coloured" key discussions.

Extracts from Mr Campbell's journals emerged during the Hutton inquiry but he said he had no plans to publish.

'Pension'

Mr Campbell told the Hutton inquiry: "I write a diary not every day but several times a week. It is not intended for publication.

"It is a series of observations about what I do and what I witness."

Mr Campbell's records of life at Number 10 have long been the subject of speculation at Westminster, with many believing they were the former communications director's "pension".

But Mr Campbell is thought to be wary of causing potential embarrassment to the prime minister with further revelations.

'Corrosive'

Mr Mulgan, who is a former head of the Number 10 strategy unit and was Mr Blair's head of policy between 2003 and 2004, said diary-keeping by insiders such as Mr Campbell threatened the quality of decision making.

He told BBC Radio 4's Look Back At Power: "There's almost nothing more corrosive to the quality of decision-making than a climate or culture in which every participant is secretly writing their diary under the table."

Asked if he knew that colleagues such as Mr Campbell were writing diaries while he and others discussed sensitive policy ideas, Mr Mulgan said: "This was known. It was not a secret. And I think it did probably colour the quality of those discussions."

Mr Mulgan said he was so concerned that he even suggested "putting in people's contracts provisions to stop them exploiting those detailed decision-making discussions for their own gain in future".

Clinton administration

Any future prime minister should make it a condition of employment "that this is not raw material for books and diaries and so on," he added.

"Those working at the heart of any government have to be able to trust each other.

"They have to be able to air ideas and proposals and thoughts, safe in the knowledge that those won't be reported outside the building.

"We were very struck by seeing the first Clinton administration in which almost everyone wrote a diary; in which within a few months of decisions being made and discussions happening, they would appear in books and newspapers."

'Focus groups'

Another former senior adviser to Mr Blair, speaking on the same programme, attacked Mr Blair's use to focus groups which he said exerted too much power.

Jon Cruddas called for a "different form of Labour government" that could break away from the "dead hand of Middle England" to confront difficult issues such as poverty and inequality.

He said Mr Blair could be forging policy "on the basis of the preferences and prejudices of focus groups or key swing voters in marginal seats to the detriment of more traditional bodies of ideas or traditions of thought within Labour."