The BBC Radio 4 Today programme said that it had obtained emails showing that Mr Livingstone's controversial equality advisor Lee Jasper orchestrated the campaign to prevent Mr Phillips becoming chairman of the new Commission for Equality and Human Rights (CEHR).

In one email, Mr Jasper called for the development of a "devastating critique" of Mr Phillips' tenure in his then job as head of the Commission for Racial Equality.

He was said to have recruited consultant Anne Kane in the summer of 2006 to spearhead the campaign. The Greater London Authority's head of diversity and performance, Audrey Young, was instructed by Mr Jasper to work with Ms Kane on the campaign.

"This will require your team to provide both full secretariat and policy research support to ensure delivery of this important project," he informed Ms Young in an email.

Ms Kane was said to have been responsible for researching articles about Mr Phillips and co-ordinating meetings with black activists who opposed him taking over the CEHR.

In a statement to the programme the GLA denied that there had been any campaign to undermine Mr Phillips and said that a previous internal inquiry into similar allegations had found that there was no case to answer. It said that, as an elected politician, Mr Livingstone was entitled to take a position in regard to matters of public policy.

"There are public differences of policy between the Mayor and Trevor Phillips. It is entirely legitimate and normal for the Mayor to express his view on such matters," it said.

However, Tony Travers, a local government expert at the London School of Economics expressed concern that it appeared that a campaign had been run to undermine a senior public official.

"If there were any suggestion that public money were being used, then clearly the District Auditor would be the right person to decide whether the spending was lawful. Then I think we could make a fair decision about Lee Jasper's position," he told Today. "I think his position is very difficult at the moment and what we need is a proper, independent examination of all these issues."