The Communication Workers Union has described Royal Mail's decision to hire up to 30,000 temporary workers as "a stupid move".
The extra staff will deal with the backlog caused by the strikes as well as helping with the Christmas rush.
The CWU has called nationwide strikes on Thursday and Friday over pay, conditions and postal reforms.
On Monday, last-ditch talks are due to be held aimed at preventing the planned industrial action.
The government has warned the CWU to bear in mind that there are alternatives to the postal system.
"Technology is providing new ways of communicating," City minister Lord Myners told CWU general secretary Billy Hayes on the BBC's Andrew Marr show.
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
"People who can't use the mail next week, Billy, are going to find other ways of communicating.
"And you are placing your members' lives and jobs at risk if you don't get back to the negotiating table."
Mr Hayes said that his union wanted "to get back to the negotiating table" and welcomed Lord Mandelson's suggestion of the use of the conciliation service Acas.
He stopped short of describing the extra temporary workers as strike-breakers.
"I think it's something that's not going to help resolve the dispute - it's going to inflame things," he said.
The Royal Mail would usually only hire 15,000 temporary Christmas staff.
But it said the extra workers would cut the impact of "unjustified and irresponsible" industrial action.
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.
Liberal Democrat leader Nick Clegg told the BBC: "I think Royal Mail are right to try to keep mail flowing and the postal service going."
"I think the people who are calling for a strike are wrong, which isn't to say they don't have legitimate concerns. But taking the whole country hostage like this is the wrong thing to do."
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.
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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