Postal workers are being offered bribes of thousands of pounds if they do not join a strike over branch closures, jobs and pensions, a union leader has claimed.

Members of the Communication Workers Union (CWU) and Unite will stage a 24-hour walkout on Thursday partly over the closure of so-called Crown offices, with postal services moved to stores such as WH Smith.

Terry Pullinger, deputy general secretary of the CWU, said it was now "do or die" for postal workers and post offices.

He told the TUC Congress in Brighton that the government and the company described the Post Office a success story. But he added: "Their idea of success is to sell Crown office buildings, put the money in the government's coffers, then outsource or franchise the service by sticking it in the back of a WH Smith, redeploy decent, union-negotiated jobs and replace them with minimum wage, zero hours contracts.

"The government has now closed the defined benefit pension scheme, probably the best funded in the country. They are using public money to bribe people to scab in this dispute, offering them £3,000 to try to weaken the resolve of the union, but they will not be successful."

Ivan Monckton, of Unite, said the government had promised to make post offices the front office of its services, but it was more like the "outside dustbin area".

He accused the government of presiding over an "ill-managed decline" of the post office.

The conference called on the Post Office board to resign and pledged full support to the union campaign.

A Post Office spokesman said: "The union's comments are misleading. The £3,000 referred to by the union will not be compromised by taking industrial action on Thursday.

"It is a conditional payment being made to employees within our supply chain business for maintaining certain quality standards during a period of change.

"Clearly, taking prolonged industrial action could make it more difficult for us to achieve quality standards, and the payments we have offered would be less affordable if quality drops and we lose revenues as a result."