The five companies -- Hays Commercial Services, TPG Post, UK Mail, Express and TNT UK -- are usually limited to providing bulk mail services for business customers.
The announcement comes as thousands of striking postal workers began returning to work after a tough weekend of negotiations between the Communication Workers' Union and Royal Mail.
Postboxes in affected parts of London were sealed from lunchtime on Friday, and there is a backlog of tens of millions of undelivered letters, which will take weeks to clear.
Royal Mail head of operations Roger Baynes said: "We've had to seal boxes because we can't responsibly keep taking letters and packets we can't move or deliver. London usually deals with close to 20m items every day. We can't let the backlog in the system keep building."
In London, around 16,000 of the capital's 29,000 postal workers were on strike. In Oxford, Coventry, Warrington, Hatfield, Chelmsford, Dartford and Colchester, 4,000 out of 134,000 employees were involved.
Postcomm said the five mail companies would be able to provide services for a minimum of 14 days if they wanted to. The regulator is taking advice on what criteria to use to decide when to end the operating period.
The Communication Workers' Union and Royal Mail will still be meeting for mediation talks at Acas this week to discuss the pay and working practice issues at the heart of the dispute.
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