The marketing body hit out at the union following the latest reports that more than 20m items of post have been left undelivered in London alone as a result of the recent strikes.

According to the union, the mail backlog is more severe than 2007's national strike, with an additional million items in limbo in Bristol, half a million in Peterborough and quarter of a million in Leeds.

In a letter to business secretary Lord Mandelson, Robert Keitch, chief of membership and brand at the DMA said it was clear businesses cannot sustain such interruptions indefinitely.

He said: "Disruption to the post will drive companies away from using mail to sell their products, which will not be particularly helpful to the Royal Mail and its employees when mail volumes are already down year-on-year.

"However, these strikes will only serve to undermine the commercial value of post as a communication channel. A significant portion of the Royal Mail’s turnover stems from direct mail."

Saul Whitford, director at direct mail printer Pepper Communications, echoed Keitch and said continuing strike action could encourage more customers to take their spend elsewhere, while Jacky Morgan posted on printweek.com such action "is crippling for DM and there doesn't seem any resolution in sight."

During the on-going dispute, CWU deputy general secretary Dave Ward accused Royal Mail of taking a "head in the sand" approach to the industry's "problems".

He said: "Management have been attacking our members through bullying-in unagreed, often unworkable, changes.

"The dismissive attitude to staff at the same time as cuts to jobs, hours and overtime and a pay freeze has made Royal Mail a dismal place to work."

The CWU now intends to carry out further action in London, Birmingham, Coventry, Essex, South Wales, Warrington, Bristol, Glasgow and Leeds all set to be implemented next week.

The union is also set to carry out national ballot of all 130,000 Royal Mail postal workers this month.