The union has warned a national strike could take place
More than 12,000 postal workers are to go on strike in a dispute over "arbitrary" job cuts and pay.
Members of the Communication Workers Union (CWU) in areas such as London, Edinburgh and Bristol will participate in the 24-hour strike.
However the Royal Mail said almost all postal services outside London would be operating normally despite the strike.
The CWU accused the Royal Mail of making "illogical and arbitrary" job cuts and called for "genuine talks".
CWU deputy general secretary Dave Ward said: "There's nothing modern in cutting jobs and hours and imposing a pay freeze. We need genuine talks on how to modernise the Royal Mail."
Mr Ward said his union had offered a three-month no strike deal in return for modernisation talks.
He further accused the Royal Mail management of failing to address the serious issues facing the industry.
But a statement issued by Royal Mail said: "The CWU's claim to have offered a three-month moratorium is deliberately misleading and is nothing more than an attempt, backed by the threat of escalating strike action, to actually halt the modernisation of the mails business."
It went on to suggest that if the process of change at the Royal Mail was halted, it would only serve to worsen the decline it has suffered in UK mail volumes.
Letters to Royal Mail chief executive Adam Crozier and Business Secretary Lord Mandelson will be delivered by the union ahead of a rally in central London later.
The Direct Marketing Association warned that the strike would damage businesses across the UK.