Postal workers in London and Scotland will take strike action over the next two days in disputes over jobs and services.
The Communication Workers Union said thousands of its members in London and Edinburgh will walk out tomorrow, while workers in parts of Scotland will strike on Saturday.
The union accused the Royal Mail of cutting jobs and services which it said broke a national agreement and threatened modernisation.
Deputy general secretary Dave Ward said: "We are now seeing cuts but not modernisation in the postal industry and there's only so long before this is going have a major impact on services.
"The CWU does not and has not blocked change. Once again we are seeing Royal Mail working against the union and failing to engage the workforce.
"The company is failing on the big picture. Machinery is not being brought in and deliveries have not been redesigned.
"We are offering Royal Mail and the Government a three month no-strike deal if they fulfil the agreement to engage the CWU over modernisation and move to get the company on a sound footing for the future."
A company spokesman said: "The Royal Mail urges the CWU to call off its planned action and honour, as we have done, existing modernisation agreements signed by the union leadership in the presence of the TUC in 2007 and which has led to the successful introduction of new ways of working in units across the UK.
"All the union will achieve by strike action is to hurt customers and undermine our drive to build a successful future for this business and its people."
Royal Mail said the three largest offices in London were not involved in the action, adding that management volunteers would help deliver post in the capital during the strike.
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies