Thousands of women council workers across Glasgow plan to bring the city to a standstill this week in what is believed to be the biggest equal pay strike seen in the UK.

More than 8,000 workers, mostly women who have never been on a picket line, will take part in the two-day action that starts next Tuesday and will affect homecare, schools and nurseries, cleaning and catering services across the city.

While Glasgow city council insists there is no justification for the planned disruption, which it says will jeopardise the care of its most vulnerable residents, unions say that a failure of negotiations has left the women with no choice but to strike and make visible the decades-long pay discrimination that has affected this largely unseen workforce.

The dispute stems from 2006, when a new job evaluation scheme was introduced by the then Labour-run council, with the aim of addressing gender pay inequality. Instead, say the women affected, it entrenched discrimination by paying female-dominated jobs such as catering and cleaning less than male-dominated jobs such as refuse collection, despite them being deemed of equal value, because of a complex system that penalised people working split-shifts and irregular hours.

The scheme also built in a three-year payment protection for men who lost out on bonuses, which was only last year ruled discriminatory by the court of session in Edinburgh.

A 12-year battle has been fought through the tribunals and courts. Many hoped it would be expedited when, after decades of Labour control, the SNP won the council elections in May 2017 on a manifesto that promised to settle the claims.

Describing the negotiations that followed as a sham, the lawyer Stefan Cross, who represents 8,000 of the claimants, said the council had repeatedly refused to engage with any of the underlying legal issues or state its own position across nearly 12 months and 21 meetings, before ditching a timetable suggested by the claimants and stating it would provide them with an offer in December.

“It’s just not good enough. An offer should be the product of negotiations, not the start. The women themselves demanded a ballot for strike action after that because their employer was refusing to negotiate,” he said.

The ballot results were overwhelming: 99% of Unison members and 98% of GMB members were in favour of action. But critics accuse these unions – who they believe have reinforced the industrial power of men at the expense of women for generations – of manipulating an equal pay dispute to score points against the new SNP administration.

The leader of the council, Susan Aitken, reaffirmed her personal commitment to equal pay and questioned the motivations of the unions leading the strike in an interview with Glasgow’s Evening Times last Thursday.

“Had the trade unions employed their industrial muscle on this issue long before now we wouldn’t be where we are now. They let the women down for a long time,” she said.

Shona Thomson, a homecare worker for 18 years and also her GMB branch secretary, said it was patronising to suggest the women were being exploited by their unions. “It’s the cleaners, carers and caterers who pushed for the strike,” she said. “We’ve been shouting about it since last year, and the union listened to us. We’ve got to the end of our tether with it.”

In August, Scotland’s local government spending watchdog described as unprecedented the financial burden facing the council if it paid out on the claims, raising the prospect of Scottish government intervention. Unions estimate that the final bill could be close to £1bn, although the council has described this figure as “plucked out of thin air”.

GMB organiser Rhea Wolfson said the reason for council intransigence was obvious. She said: “We were negotiating with the same officials who were advising their Labour predecessors to continue litigation against us for years.”

But Aitken told the Evening Times there was a frustrating lack of clarity about the unions’ demands, as well as shifting public rhetoric around the reason for the strike.

“I’m not entirely sure the women know the basis on which they are striking. I’m not convinced they actually know the demands made on us. They are not ‘pay up now’. They deserve to know that the union is not asking for it go faster. There is an amount of work to be gone through and we will go though it as quickly as possible.”

For Thomson, the spur for striking goes beyond the headline issue of equal pay. “Low-paid woman are always fearful about losing their jobs, but we realised that we’re worth more than this,” she said. “It’s not just about equal pay but the changes we’ve seen, the increased workload, pressure, split shifts. The majority of these women are over 45 and have been doing their jobs for 20 years.

“We’re not seen by the city but we keep the cogs turning, working behind the scenes. I love my job but I’ve come to realise that I’m a strong woman who can speak out and say, ‘this isn’t right’.”

• This article was amended on 15 October 2018 to correct the date of the strike. It is due to go ahead on Tuesday 23 October rather than Tuesday 16 October as previously stated.