Lord Mandelson said unions had to help stop Royal Mail's decline
The Business Secretary Lord Mandelson has rejected calls to intervene in the latest postal strikes.
Union leaders have accused ministers of "sulking" because their plans to "privatise" Royal Mail had failed.
But Lord Mandelson said the Communication Workers Union should "wake up" to prevent Royal Mail's further decline.
The walkouts in England and Northern Ireland were triggered by a row over modernisation plans.
Lord Mandelson said: "Time and again in the past, the CWU has asked ministers to intervene in their disputes and their strikes to frustrate Royal Mail modernisation.
"I have instructed this will not happen. It is time for the union to wake up to the need for change to stop the Mail's further decline."
Members of the union have been staging a series of local 24-hour strikes.
They are due to continue into next week, when the union is planning to ballot its members on holding a national strike.
Speaking earlier, the leader of the CWU, Billy Hayes, said ministers "appeared to be on strike themselves".
Mr Hayes said: "There hasn't been one government minister, at any level, talking about the strikes - which have disrupted services."
He told BBC Radio 4's Today programme: "It seems to us that some government ministers are sulking because they haven't yet managed to privatise Royal Mail.
"This is a company that they own, and they don't seem capable of saying one single word.
"Ministers appear to be on strike themselves."
19 August: Birmingham, Coventry, London, Essex, Peterborough, Bristol, Leeds
20 August: Peterborough
21 August: Peterborough, Kings Lynn
22 August: Boston, Carrickfergus
24 August: Skegness, Huntingdon
In June, Lord Mandelson, announced that plans to sell off part of Royal Mail had been delayed amid opposition from many Labour backbenchers and the failure to find a suitable buyer.
Mr Hayes denied the union was trying to block changes at Royal Mail.
Modernisation shouldn't mean "reducing services to the public", he said.
"Royal Mail is trying to bring in institutional changes without consultation," he added.
"There's going to be more strikes, we've got a situation where Royal Mail is cutting people's pay, taking people off duties without agreement."
Earlier this year, the company warned it was in a "very tight financial position" and had to cut costs.
In 2008, all four parts of Royal Mail were profitable for the first time in almost two decades.
A spokesman for Royal Mail said the strikes were having little effect.
He said: "Well over 90% of our people - and more than 90% of our operational units - will be working normally this week."