The accusation was made in yesterday's Daily Mail which said that union general secretaries would be "as mute as Trappist monks" during the hustings.
The paper said that leading trade unionists agreed at a meeting two weeks ago to remain silent in case they detracted from the support the party seems to be attracting in Middle England.
Senior union officials described the front-page story as "rubbish", arguing that most of the time the media was not interested in their views on the general election.
Bill Morris, general secretary of the Transport and General Workers' Union, said it was full of "speculation and misrepresentation" and challenged the newspaper to grant him space to tell readers why they should be supporting "New Labour".
He said: "Far from keeping silent during this election campaign I have a full schedule of commitments to take part in meetings around the country." Mr Morris was due to be interviewed on television last night to give his views on the Labour manifesto.
Rodney Bickerstaffe, leader of public service union Unison, denied there had been any agreement and said he would be taking part in the hustings. Mr Bickerstaffe has, however, been more circumspect about his support for the Labour Party than most union leaders, because Unison has two political funds, one of which is entirely independent of Labour.
John Edmonds, general secretary of the GMB general union, said it was "ridiculous" to suggest there had been any conspiracy. "This election is about politics, not union leaders. We will be doing our best to help Labour," Mr Edmonds said.
Lew Adams, leader of Aslef, the train drivers' union, who was among those said by the Mail to have taken the vow of "Omerta", would be launching a campaign in his union's journal urging members to vote Labour. Mr Adams wrote in a recent edition that he was sceptical about Labour's promises to reform employment law.
Roger Lyons, leader of the MSF manufacturing union, was on holiday yesterday, but when he returned would be meeting his members in marginal constituencies, a spokesman said. "That is hardly the activity of a Trappist monk."
Despite their denials however, it is clear that union leaders are not as "high profile" as they have been in previous elections. That is partly because they acknowledge they could be more of an electoral hindrance than help. Tony Blair, the Labour leader, is also keen to put considerable distance between the party and unions.
Nevertheless, it is unlikely there has been any formal agreement to keep silent.
What has been clear over the last few years is that there has been an unspoken pact between the two wings of the movement over policies. The party has told unions they could have their union recognition laws and a national minimum wage provided they did not wield their block vote to disrupt the change from Old to New Labour.
The Labour leadership has been keen to keep unions on board because of they provide most of the party's funds. Union officials are also active in canvassing and provide administrative back-up.Reuse content