Dave Ward, CWU: "There are external forces... attempting to undermine talks"
A nationwide postal strike will go ahead on Thursday and Friday, the Communication Workers Union (CWU) says.
The action, to begin at 0400 BST, comes amid a row over pay, conditions and Royal Mail's modernisation plans.
The CWU has said it will also announce further strike dates - expected to be revealed on Thursday afternoon.
The union criticised Royal Mail bosses and Business Secretary Lord Mandelson for not getting involved in talks to end the dispute.
Mail centre staff and drivers will strike on Thursday while delivery and collection staff will take action on Friday.
Government sources told the BBC the strike was "something of a tragic matter" and revolved around a dispute over implementing working practices and changes agreed two years ago.
Business Minister Pat McFadden MP: "No clear reason has been given for the strike"
Meanwhile Royal Mail condemned the strikes which it said were "wholly unjustified".
It had earlier issued a last-gasp plea for a "strike-free period of calm" in the run-up to Christmas, saying industrial action would bring "huge damage and distraction" and cause "pain and disruption" for customers.
The CWU's deputy general secretary, Dave Ward, said that this letter was "substantively different" from the from of words drawn up by Royal Mail negotiators.
"The form of words drafted between the negotiating teams last night was precisely that - a form of words that may have allowed the two sides to keep talking and call off the strikes."
STORY SO FAR...
Postal workers, especially in London, have been holding intermittent one-day strikes for months in a row over the way Royal Mail is to be modernised
Earlier this month postal workers voted three to one in favour of nationwide industrial action (though Royal Mail said 60% of the total number of postal workers working in the UK did not vote to strike)
The CWU set dates for the first nationwide postal strikes in two years
Last-gasp talks failed to reach an agreement and indeed the split between the union and Royal Mail management became more acrimonious
Royal Mail said the agreement was still available to be signed, and to prevent the strikes being necessary.
The CWU had "no confidence or trust" in either Mr Higson or Royal Mail's chief executive Adam Crozier, Mr Ward said.
And it accused Lord Mandelson of working "hand in hand with Royal Mail" to deliberately undermine the talks, saying the minister had privately expressed his doubts about the capabilities of Royal Mail's management.
Billy Hayes, the CWU's general secretary, labelled Lord Mandelson the "minister without responsibility".
The prime minister had urged unions and managers to agree a deal saying a strike would be "counter-productive".
Earlier, in the Commons, David Cameron accused Gordon Brown of lacking the "courage and leadership" to intervene in the postal dispute to prevent two days of planned strikes.
The decision to go ahead with strike action came after more than 30 hours of talks between Royal Mail and the union this week.
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