The largest union representing school staff in Scotland has described what council leaders said was their “best and final” pay offer to workers, to avoid strikes next week, as “too little, too late”.

The Convention of Scottish Local Authorities (Cosla) said its “significantly improved in-year offer” would mean workers on the Scottish local government living wage see an in-year uplift of about £2,000, or almost 10%.

But Unison, one of three unions involved in the negotiations, said strikes will go ahead as it has rejected the deal.

It means school staff including cleaners and caretakers will walk out for three days from Tuesday.

In a statement Cosla resources spokeswoman Katie Hagmann said she was “extremely disappointed” Unison had rejected the latest offer and found their actions to be “totally unacceptable”.

However the union claims the vast majority of local government staff are only being offered 0.5% more than Cosla’s original offer, which was made almost six months ago and was rejected by members.

It also says a commitment to pay the lowest paid workers £15 per hour is “too vague”, and because no new money has been found to fund the improved offer it will “inevitably lead to more cuts to jobs and services”.

Unison Scotland head of local government, Johanna Baxter, said: “This revised offer is far too little, too late. Strikes will therefore proceed next week. We cannot agree to a pay offer that will result in further cuts to our members’ jobs and the services they provide.

“It has taken Cosla six months to send us a revised pay offer which, for the vast majority of staff, is an increase of only 0.5% in-year. These are not well-paid staff, they are on less than the Scottish average wage and it is simply not acceptable.

“Far from learning the lessons of last year’s dispute the situation has been worse this year, caused further delay to local government workers’ pay during a cost-of-living crisis and created uncertainty for parents.

“This is no way to conduct industrial relations.”

Chairman of Unison Scotland’s local government committee, Mark Ferguson, said: “The offer is still below the rate of inflation, meaning that local government workers are, once again, being asked to take a real-terms pay cut which they can ill afford during a cost-of-living crisis.

“Our members have been left waiting for an improved pay offer for months after their pay uplift was due, and right up until the eve of mass school closures, whilst Cosla and the Scottish Government have prevaricated over who will find the additional money needed to fund any improved offer and where the money will come from.”