Mail deliveries could be disrupted as postal workers in west Oxfordshire begin a three-day strike.
About 100 postal workers in Witney and Carterton are set to continue the action on Friday and Monday.
The official strike follows an unofficial walkout last month. The action was sparked by the suspension of a postman and allegations of bullying.
Royal Mail said it was drafting in managers to help minimise any disruption to customers.
A ballot was held two weeks ago after claims post office bosses broke local and national agreements in treatment of staff.
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We are extremely disappointed Royal Mail staff at Witney and Carterton delivery offices have decided to go ahead with strike action
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Staff at both centres voted in favour of a strike to take place last week but this was rescheduled to avoid disruption to deliveries during GCSE exam results week.
A spokeswoman for the Communication Workers Union said: "Because of the GCSE results coming out last week we didn't want to cause disruption to the children."
Talks held between the union and Royal Mail on Wednesday failed to reach an agreement.
In a statement, the company said: "We are extremely disappointed Royal Mail staff at Witney and Carterton delivery offices have decided to go ahead with strike action.
"Over the past two weeks we have met with our people and with representatives from the Communication Workers' Union on numerous occasions in a bid to address concerns and resolve the matter."
Royal Mail said it had agreed with the union that an independent review of industrial relations at Witney and Carterton delivery offices would be carried out.
The company apologised to any customers who experience disruption.