Post Office workers will walk out on Monday in their ninth strike since Easter in a battle against government plans to franchise out 70 high street branches.

The walkout by 4,000 workers in 373 crown post offices – the large high-street operations selling everything from stamps to financial services – has been timed for the last Monday of the month to cause maximum disruption.

The Communication Workers Union (CWU), which represents post office staff, claims the franchise plans will lead to 1,500 job losses and reduced customer service. The Post Office insists there will be no compulsory redundancies among staff who work at the affected branches.

Andy Furey, CWU national official, said: "It's clear that this dispute is growing in both public and political support while the determination of Post Office staff is unwavering. Tens of thousands of customers have signed petitions against closures and a dozen political bodies – including the Scottish parliament, Greater London Authority and local councils – have passed motions against the company's proposals. In fact we can't find anyone who is in favour of them."

Local councils that have voted through motions against the closures include Manchester city council and Hastings borough council.

The Post Office, which is state-owned and receives £1.3bn of taxpayer subsidy, has said franchising out branches to retailers is essential to securing the future of the service.

Kevin Gilliland, the Post Office's sales director, has said the crown network of the biggest post offices across the country is "losing £40m a year of public money" and "must change the way they operate to ensure long-term viability".

The Post Office is a distinct state-owned company from Royal Mail, which the government is aiming to privatise before the end of the year.