Members of the Communication Workers Union (CWU) will walk out for 24 hours on October 16 over a long-running row about allowances, threatening major disruption to mail collections and deliveries.
A strike last week cost the Royal Mail up to £10m and took the company five days to clear the backlog of post.
The CWU action will coincide with a stoppage by council workers in London in a separate pay dispute. Coordinated strikes by unions representing workers from different sectors is highly unusual, but a growing number of disputes are breaking out in the capital.
Striking postal workers will stage a lobby outside the headquarters of the Department of Trade and Industry and will hold a rally in central London.
CWU deputy general secretary Dave Ward said: "We have chosen the same day as the council workers' strike to raise the issue of low pay across the public sector."
The Royal Mail has warned that a strike will not lead to an increase in its offer but would merely add fresh financial pressure onto the organisation.
The CWU is seeking a London weighting allowance of £4,000 a year. The Royal Mail has offered an increase of £300, which will raise the outer London weighting to £2,667 and inner London weighting to £3,784.
London council workers are also striking over weighting allowances in the capital. Mr Ward added: "All we are looking for is a fair deal. Prices of accommodation, transport, food and entertainment are vastly higher in the capital than the rest of the country, and our members need levels of London allowances that reflect that position.
"Otherwise we are accepting that our London members should have a lower standard of living than other postal workers.
"Last week the business was fined £7.5m by the regulator for offering an unsatisfactory quality of service. That money could have been invested in London staff. If they are treated fairly and reasonably, they will respond in a positive way that will be reflected in quality of service."
Mr Ward said the strike was motivated "not by greedy London, but by needy London".