The action is part of a series of UK postal strikes
Thousands of postal workers across London have walked out for 24 hours in a row over jobs and pay cuts.
Members of the Communication Workers Union (CWU) began the strike from 0300 BST in delivery offices and collection hubs across the city.
The union is protesting against job and pay cuts which it says were imposed by Royal Mail "without consultation".
Royal Mail 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 and London.
It is the 11th 24-hour postal strike to take place in London since June.
A CWU spokeswoman accused Royal Mail of "imposing change without agreement". The union wants the management to include it in discussions about modernising the service.
She said Royal Mail was "cutting jobs, cutting pay, cutting overtime, cutting hours and at the same time forcing people to change the way they are working".
The union said up to 20 million items of mail remain undelivered in London sorting offices due to the strikes.
CWU divisional representative Martin Walsh said: "Royal Mail has introduced revision after revision, which is failing.
"[Workers] cannot clear their normal workload, let alone the backlog that's within the system."
On Tuesday 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."
Union members in south-west London and Wimbledon are due to stage a further strike on Thursday in action expected to last until Friday night.
London-wide walkouts will continue until Saturday, when CWU members across England will take industrial action.