The CWU said the business secretary's comments "would damage the talks".
"While the CWU and Royal Mail management are working hard to reach an agreement, Lord Mandelson is misrepresenting the facts to Parliament and undermining any progress being made," it added.
If the strikes did go ahead for a prolonged period, contingency plans for delivery of hospital appointments and medical test results were being drawn up, the Lords were told.
Earlier it emerged that the Ministry of Defence may charter extra aircraft to ensure serving troops get their Christmas post.
Defence Secretary Bob Ainsworth told the Commons there would also be "increased resources dedicated to mail movement" within Afghanistan but added "prolonged industrial action" would "inevitably" hit deliveries.
Royal Mail is in dispute with the CWU over pay, conditions and modernisation plans.
The company says it is trying hard to reach an agreement.
'Unjustified and irresponsible'
Acas has appointed a trouble-shooter who is on stand-by to step in and facilitate peace talks in the event of a breakthrough.
But BBC correspondent Martin Shankleman said Royal Mail's insistence that it would go to Acas only if the union called off its strikes was a stumbling block.
The talks between the two sides came after CWU general secretary Billy Hayes described Royal Mail's decision to hire up to 30,000 temporary workers to help deal with the effects of strike action as "a stupid move" that could inflame the dispute.
Royal Mail will be hiring double the 15,000 temporary staff it usually takes on before Christmas.
Management said the extra workers would cut the impact of "unjustified and irresponsible" industrial action.
The extra workers will deal with the backlog caused by the strikes as well as helping with the Christmas rush.
Employing extra people to do the work of staff who are on strike is illegal under employment law.
Royal Mail said the recruitment drive was not about bringing in workers to do the work of striking staff, but to ensure there were enough staff to clear any backlogs between walkouts, as well as tackle the seasonal increase in mail volume.
On Thursday, mail centre staff and drivers are planning to strike. The next day it will be delivery and collection staff.
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