The CWU says 20 million items of mail remain undelivered in London
Postal workers are staging a 24-hour strike in south-west London ahead of a city-wide walkout in a row over cuts.
Members of the Communication Workers Union took industrial action from 0700 BST in Tooting, Balham and Brixton.
A London-wide 24-hour postal strike begins at 0700 BST on Wednesday. The union is protesting against job and pay cuts imposed "without consultation".
A Royal Mail spokesman said it was "getting on with modernisation in an entirely fair and reasonable way".
The industrial action is part of a series of strikes around the UK.
A spokeswoman for the Communication Workers Union (CWU) said: "Royal Mail is imposing change without agreement.
Post workers strike for 24 hours
"It is cutting jobs, cutting pay, cutting overtime, cutting hours and at the same time forcing people to change the way they are working."
She said the union wanted Royal Mail to "sit around the table and talk about the job to be done in modernising the service".
The CWU says up to 20 million items of mail remain undelivered in London sorting offices due to recent strikes.
Union members are voting on whether to hold a nationwide strike.
Royal Mail Operations Director Paul Tolhurst said: "It is now more clear than ever that the CWU is focussing all its efforts on strike action and political posturing.
"Their claimed support for change is no more than lip service but their determination to damage services and undermine the future health of the business and the universal service is increasing."
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