85 post offices in London are expected to be affected by strike action today.
Staff are due to walk out in protest at plans to close or franchise 75 of the "Crown offices" - larger branches usually sited on the high street. Members of the Communication Workers Union say that services and jobs will be affected by the planned closures.
The action is part of a long-running row over closures, jobs and pay.
The Post Office said the 370 Crown branches were losing £37 million a year and had to be brought into profit.