Pensions action will be a women's strike, says union

Unison says 30 November strike will be dominated by females and undermine Conservative party efforts to reconnect with women voters

Dave Prentis, of Unison, said women make up nearly two-thirds of the workforce affected by the pensions reforms. Photograph: Martin Argles for the Guardian

Next Wednesday's industrial action over pensions reforms will be a "women's strike", according to the leader of the largest public sector union, as a row broke out over government claims that the 30 November walkouts will cost the economy £500m and trigger job losses.

Dave Prentis, general secretary of Unison, said the strikes by up to 2.6 million workers covering public sector jobs ranging from teachers to librarians and immigration officers would be dominated by female participants and worsen the Conservative party's problems with female voters.

In an effort to shift public opinion against the strikes and dissuade workers from taking action, ministers claimed that the economy will lose £500m and an unspecified number of jobs because working parents will be forced into emergency childcare by school closures – an estimate dismissed by one respected thinktank as "economic nonsense".

However, in an interview with the Guardian Prentis said attempts to portray unions as "old-fashioned" monoliths would be exposed as inaccurate.

Unison has 1 million female members and union leaders believe that women make up nearly two-thirds of the workforce affected by the pensions reforms that have triggered Wednesday's day of action.

"This will be a women's strike," said Prentis. "It is women who voted for this. We have a million female members and it is their decision, they wanted to take this action."

Prentis – who also chairs the public sector group at the TUC – said the strike would not conform to critics' allegations of dinosaur-like behaviour. "The rightwing press brand this as an old-fashioned trade union dispute, linking it to strikes in the past. But this is different. People who provide our local government services, our health services, people who care for us, are saying they cannot take it any more and 75% of these workers are women."

Amid reports that internal Tory polling shows women in skilled manual jobs are deserting the party, Prentis said the strikes would poison the coalition's rapport with female voters. "I think the government is beginning to realise that the overwhelming majority of people taking action will be women. They are worried about women moving away from the coalition."

The TUC is reluctant to discuss further action after 30 November, amid doubts that a deal can be reached by the end of December, but the three largest unions – Unite, Unison and the GMB – have talked openly about a long-running campaign of walkouts and protests.

Prentis declined to rule out more strikes if talks fail. "Members did not just vote for action on 30 November. They have voted for action full stop. If our members believe that they want to take further industrial action if an offer from the government is not good enough, then we are in a position to take that action."

Unison balloted 1.1 million members in local government and the health service, including nurses, school dinner ladies, nurses and physiotherapists.

Prentis warned that cover on local government and health services would be kept to a minimum. He said: "People will be inconvenienced. To think that we can take strike action and not inconvenience people, that would not make sense."

With less then a week to go before the biggest walkout in decades, the ministers in charge of pensions negotiations, Francis Maude and Danny Alexander, said the strikes would impose a "significant hit to the economy at a very challenging time" as they urged public sector staff to defy their unions and turn up to work next Wednesday. Alexander, the chief secretary to the Treasury, said he might take his daughter into work due to school closures.

The Treasury said the planned walkout would curb output in the public and private sectors, although the figures do not factor in savings from a swath of the workforce losing a day's pay.

A Treasury spokesman said: "The Treasury has a number of economists and they have calculated this. This is a reasonable worst-case scenario. We don't know what the impact of the strikes will be."

Maude, the minister for the Cabinet Office, said redundancies were likely because there was a correlation between economic output and loss of employment – though he was unable to put a figure on the numbers.

"Exactly what that relationship is is very hard to anticipate but if we lose a big chunk of output it is hard to see how that does not translate into fewer jobs," he said.

According to the government, the biggest impact will be caused by thousands of school closures, forcing parents to work from home, make childcare arrangements or take their children to the office. Jonathan Portes, director of the National Institute of Economic and Social Research, dismissed Maude's claim as "economic nonsense". "There is no direct reason why a strike next week will translate into job losses," he said. "Clearly, a prolonged period of industrial action which had a medium-term damaging effect on the economy would be different."

Jon Trickett, the shadow cabinet minister, expressed fears Treasury civil servants were being used to feed a Conservative "propaganda machine". He said: "I really do think Francis Maude needs a lesson in mathematics. Everybody knows you have to take the pluses and the minuses to calculate the real cost of something like this. This is a political stunt hiding behind inaccurate mathematics."