Shadow business secretary Kenneth Clarke said the Conservatives would privatise the Royal Mail if it wins the next general election.
Mr Clarke said the company needed private capital and private management to bring it up to date and change it from being "old-fashioned".
The shadow minister told BBC Radio 4's Today programme that the Royal Mail was "broke" and its future had to involve a change of culture to stop business "draining away".
"We propose to bring in private capital, assuming it is not in too disastrous a state by next May."
Mr Clarke said the Royal Mail was becoming a "total disaster", attacking the Prime Minister for "changing his mind" over the stalled part-privatisation plans.
"The Government has done nothing about the Royal Mail and we now have what could be a truly disastrous strike."
CWU general secretary Billy Hayes told the same programme that the union was "quite happy" to hold talks at Acas, but urged Royal Mail chief executive Adam Crozier to become involved.
He also repeated his strong criticism of Lord Mandelson, describing him as the "minister without responsibility".
Mr Hayes bumped into Royal Mail's operations director Paul Tolhurst at the BBC studios and the two men had a cup of tea together.