The CWU has announced further action in London, Birmingham, Coventry, Essex, South Wales, Warrington, Bristol, Glasgow and Leeds all set to be affected next week.

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.

The union's deputy general secretary Dave Ward has 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 union is now set to carry out national ballot of all 130,000 Royal Mail postal workers this month.

Royal Mail has condemned the latest strike action. Paul Tolhurst, Royal Mail's operations director said: "We urge the CWU to abandon strikes and the threat of strikes, and focus on providing customers with the service they need and expect, rather than planning to hurt them with the threat of more strikes."