After last-ditch talks between the Communication Workers Union (CWU) and Royal Mail failed last night, a 48-hour nationwide postal strike was due to begin at midnight. The action comes amid a row over pay, conditions and Royal Mail's modernisation plans.

The CWU plans further strikes and said that it would give details later in the week. It criticised Royal Mail executives and the Business Secretary, Lord Mandelson, who it said was working "hand in hand with the Royal Mail" to "undermine the dispute".

Mail centre staff and drivers will strike today while delivery and collection staff will take action tomorrow.

Royal Mail has condemned the strikes as "wholly unjustified". It had asked for a "strike-free period of calm" in the run-up to Christmas, saying that 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 while progress had been made in discussions last night, they had been vetoed by the Royal Mail's managing director, Mark Higson. Mr Ward said the CWU had "no confidence or trust" in Mr Higson, Royal Mail's chief executive, Adam Crozier, or Lord Mandelson.

Royal Mail said last night that the CWU's claims were "outrageous" and denied that it had vetoed proposals.

Lord Mandelson expressed "deep regret" that the strike was going ahead. He said it was "not in the best interests of the company, the workforce or the hard-pressed consumers and businesses that depend on Royal Mail".

A source close to the Business Secretary said: "The union wants Peter to intervene to unpick change that has already been agreed. He is not going to do that. By directing fire at him they are trying to hide the fact that there is no good or logical reason for this strike."

Contingency plans for delivery of hospital appointments and medical test results have been made in case strikes continue for a prolonged period.