A fresh row over the postal strikes broke out today when union leaders confirmed plans for a national ballot in the bitter dispute over jobs, pay and services.
Members of the Communication Workers Union will start voting next Wednesday on whether to back a nationwide walkout, with the result expected on 30 September - while Labour is holding its annual conference.
Royal Mail attacked the decision to press ahead with the ballot as "wholly irresponsible", as talks between senior management and the union leadership are still being held.
The news came as fresh strikes were held today in London, worsening the huge backlog of mail building up across the UK, as revealed by the Press Association yesterday.
A Royal Mail spokesman said: "The union has been saying for weeks it would call a national strike ballot but a decision to go ahead in spite of the talks underlines the union's intent to further damage the business and its customers rather than genuinely seeking an end to the current localised disputes.
"The ballot is simply the latest attempt by the CWU to oppose the essential modernisation of Royal Mail on the ground, despite its public claims to support change, and shows the CWU's determination to renege on existing agreements on change, including pay and modernisation, which the union's leadership signed in the presence of the TUC.
"Royal Mail remains committed to talking to the union and urges it not to call our people out on further strikes which can only damage the business and its ability to continue to provide the one-price-goes-everywhere universal service."
Royal Mail said it had held more than 60 meetings with the union over recent months.
Paul Tolhurst, Royal Mail's operations director, said: "Royal Mail is getting on with the essential changes the union has signed up to in a fair and reasonable way, while the union continues to try to back-pedal from its commitment to modernisation and impose a veto on the progress.
"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.
"With mail volumes now falling by around 10 per cent a year, the union needs to recognise that customers have a choice in today's open communications market."
The CWU said the mail backlog was now bigger than at the height of the national strike of 2007, with well over 20 million items backed up in London, one million items in the Bristol area, half a million in Peterborough, and a quarter of a million in Leeds.
Deputy general secretary Dave Ward said: "Royal Mail's head-in-the-sand approach to the problems in the mail industry is now severely damaging services for customers, with backlogs bigger than in the national strike of 2007.
"Management have been bullying our members into 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.
"We are successful at bringing in change by agreement. Royal Mail needs the backing of postal workers to make the company successful and make change work.
"We want them to see sense and negotiate agreements which make change work for the company, staff and customers."