Postal union leaders are due to announce dates for a series of strikes in London in a dispute over pay.
A postal dispute in London is a step closer
On Tuesday, union officials in London endorsed a decision by the national executive of the Communication Workers Union (CWU) to take industrial action in a row over London weighting.
In a ballot, London workers had voted in favour of walkouts by about 11 votes to four.
Last week, postal staff voted to reject a walk-out as part of a national dispute over pay and conditions by a margin of less than 2,000 votes.
The CWU, which has about 30,000 postal members in London, wants an increase in the London allowance to £4,000 a year.
Currently workers can earn between an extra £1,500 and £2,700.
The Royal Mail is offering an increase to £2,860 a year in outer London and £3,784 in inner London.
Any strike in London would have a huge impact on mail deliveries across the
Agreement has still to be reached on the national pay dispute, despite the vote against striking.