Unite issued a categorical denial to PRWeek: "At no point was the communications department requested to undertake any irregular activities against any party or individual, and nor would it do so if ever such a request was made," a spokesman said.
Yesterday, The Sunday Times alleged a Labour investigation into vote-rigging in its Falkirk constituency party may have been undermined by union officials.
The paper claims to have seen more than 1,000 emails sent to the police, including evidence that union officials had written a retraction letter from a witness involved with the inquiry.
The Sunday Times also printed an email it says was sent by Unite director of legal service Howard Beckett in June stating: "Comms will prepare the nasty stuff we know of individuals in the Labour Party, but this will not be used."
A spokesman rejected any implication of wrongdoing by its comms team, saying: "We did everything we could to support our members under investigation, as would be expected of the union. But this did not extend to negative briefing against the Labour Party or any individuals within it."
The emails were reportedly obtained by lawyers working for Ineos, the owners of Grangemouth, a petrochemical plant at the centre of a recent industrial dispute.
They centre on Labour’s Falkirk constituency party chairman Stevie Deans, also Unite convenor at the plant.
The Sunday Times story has prompted calls for Labour leader Ed Miliband to reopen the inquiry, which was closed last month.
However, Unite claimed it was the subject of "unjustified attacks" in relation to the selection process and said the coverage was the result of a "media witch-hunt".
Initial accusations over vote-rigging led to the resignation of Tom Watson as election co-ordinator for Labour.