The postal workers leader today accused Royal Mail of having a "cynical" plan to sideline the union involved in a bitter dispute over jobs and pay as the company braces itself for a national strike.
Billy Hayes, general secretary of the Communication Workers Union, also told Lord Mandelson he hoped the Business Secretary was not seeking "revenge" for failing to push through part-privatisation of the company.
An internal document suggests Royal Mail bosses have full government support for implementing modernisation plans with or without the agreement of the Communication Workers Union (CWU), it has emerged.
The document came to light after the CWU announced two 24-hour nationwide strikes next week as the long-running row deepened.
The Royal Mail strongly denied that senior managers had seen the document and said it had no plans to derecognise the union.
But Mr Hayes repeated today that he believed the document was genuine, adding it was "more worrying" that Lord Mandelson seemed to know what was in it.
"I hope Peter Mandelson is not sitting there, thinking this is my revenge because I could not persuade my parliamentary colleagues to part-privatise the Royal Mail," Mr Hayes said on BBC Radio 4's Today programme.
Mr Hayes said Lord Mandelson had ruled out the involvement of Acas in trying to break the deadlocked dispute.
The union leader said it was "incredible" that the Royal Mail, and now the Government, were not prepared to agree to an independent conciliator being brought in.
"It seems like a cynical attempt to sideline the union."
A Royal Mail spokesman said of the document: "No member of the board or the senior management team at Royal Mail has seen, or is aware of any such presentation.
"Royal Mail's policy and strategy in relation to the current dispute with the CWU is to reach agreement so that the CWU calls off its damaging and irresponsible strikes.
"For the avoidance of any doubt Royal Mail has never had any strategy to derecognise the CWU and nor would we seek to do so."
The first 24-hour strike will see mail centre staff and drivers walk out next Thursday, with delivery and collection staff striking the next day.
Coming in the wake of a series of regional Royal Mail stoppages, the strikes will cause huge delays to deliveries to homes and businesses around the country.
Royal Mail's managing director, Mark Higson, said the CWU's strike was an "appalling and unjustified attack" on customers and showed a "reckless disregard" for everyone who depended on the company.
Lord Mandelson branded the move "suicidal", adding: "I very much regret this decision by the CWU. Taking industrial action will not resolve this dispute. It will only serve to drive more customers away from Royal Mail. One thing this company cannot afford is strikes and industrial action.
"We are, of course, in frequent contact with both management and the union. Our message to them has been clear: put your customers first."