Royal Mail will recruit up to 30,000 temporary staff to deal with upcoming strikes by postal workers and the Christmas rush, the service has said.
The Communication Workers Union (CWU) has called national strikes on Thursday and Friday over pay and reforms.
Royal Mail said it would hire twice the usual number of extra pre-Christmas staff to cut the impact of "unjustified and irresponsible" industrial action.
But the CWU described the decision as "a stupid move".
"I think it's something that's not going to help resolve the dispute - it's going to inflame things," CWU general secretary Billy Hayes told the BBC.
But speaking on the Andrew Marr programme, he stopped short of describing the temporary workers as strike-breakers.
EXPECTED STRIKE DISRUPTION
22 October: Limited processing, movement and collection of mail
23 October: Limited delivery and collection of mail
Some managerial staff and contract drivers will be brought in to move mail but services will be "very limited"
Special Delivery and Royal Mail Tracked items will still be delivered on strike days
Post Office branches will be open as usual although collections will be affected
Parcelforce will operate as normal as it has its own collection, distribution and delivery network
Customers should check the Royal Mail website or call 08457 740 740 for the latest service information
Source: Royal Mail
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 as well as tackle the seasonal increase in mail volume.
Two 24-hour nationwide strikes have been called so far, on 22 and 23 October. On the first day, mail centre staff and drivers will strike. The next day it will be delivery and collection staff.
Meanwhile, Business Secretary Lord Mandelson said he would have no problem with going to the conciliation service Acas to solve the dispute.
He said he was beyond anger at this strike - just sad at the tragedy for Royal Mail.
Chief Executive Adam Crozier said he was "absolutely determined" to do everything he could to minimise delays to the public, adding that around 85,000 people had applied for temporary Christmas work.
He added: "We are continuing to urge the union to halt its appalling and unjustified attack on customers.
"Every year, Royal Mail recruits thousands of additional, fully vetted, temporary staff as part of the operation which successfully delivers the Christmas mail. This year we'll have twice as many people on board, and we'll have them in place much earlier in the autumn."
'Not that simple'
Mr Hayes said the decision to bring in extra workers was a sign of a management that was almost a dictatorship.
"This is about a culture of management that seems to think in a democracy that the workforce have to do just what they're told," he said.
"What I'd sooner Royal Mail be doing is negotiating seriously about trying to find a resolution to this dispute."
Postal economist, Ian Senior, expressed doubts about the ability of the temporary staff to clear the backlog of mail.
"You can't pluck people out of a job centre and say 'right, now be a postie'. It's not that simple," he told the BBC.
"If you look at what happens in sorting offices, the equipment there is very sophisticated and you need to know how to use it."
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