The union gave the go-ahead for the latest round of strike action after the mail operator and the CWU failed to reach an agreement during resolution talks yesterday.

More than 120,000 postal workers will walk out across three days, just a week after the initial two-day national strike that resulted in an estimated backlog of 30m letters.

In a stark warning for the direct mail sector, CWU general secretary Billy Hayes told the BBC: "I can see the strike action increasing now. I don't think we're going to put up with this messing about."

Key figures from the direct mail industry have voiced concerns about the effect ongoing postal strikes are having on their businesses.

Yolanda Noble, chief executive of Dsicmm, said she was "absolutely furious" that the problem had yet to be sorted.

She said: "It is bad news and the government really needs to get involved, while Peter Mandelson needs to stop sitting on the fence. If Royal Mail continues like this, it will ruin its own business."

Robert Keitch, chief of membership and brand at the Direct Mail Association, added it would be "a complete sodding disaster" if the current talks were to break down.

A CWU spokesperson told PrintWeek: "Strike action is always a last resort. Workers do not get paid when striking, but it is a proportionate response to the attack from Royal Mail management. We hope to reach a resolution as soon as possible."