Schools across much of Scotland are set to close next week after a union rejected the “best and final” pay offer from council leaders.

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 around £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.

School staff including cleaners and janitors will walk out for three days from Tuesday.

The PA news agency understands Unison has rejected the offer but will put it before members, suggesting that they also reject it.

An extra £90 million was found for the latest offer, including the £10 million previously allocated by council leaders in the previous deal offered to unions on September 13.

Cosla said the new offer was made on condition that the unions take it to their members for full consideration and that strikes are suspended with immediate effect whilst they do so.

Its resources spokeswoman Katie Hagmann said: “This really is our best and final offer, we have nowhere to go after this.

“We have gone beyond our limits in a bid to satisfy our workforce and avert next week’s strikes.”

Unions have warned more than three-quarters of schools will close if the strike goes ahead, affecting 26 of Scotland’s 32 council areas.

Unison, Unite and GMB Scotland rejected the previous “measly” pay offer from Cosla, saying it would lead to an increase of 38p per week for the lowest paid.

Cosla had said the previous offer would have provided at least a £1,929 increase in annual salary for workers by January 1, and the lowest paid would have seen a 21% rise over two years.

Ms Hagmann said on Thursday that no new money had been identified for the latest offer, and there will be “delays to programmes and projects within communities” to meet the pay demands.

She said: “Despite the extreme difficulties this presents us with, Scotland’s council leaders have listened and acted on the ask of our trade union colleagues to get us to this position today.

“We can now make a significantly improved in-year offer meaning those workers on the Scottish local government living wage will see an in-year uplift of around £2,000, or almost 10%.

“I cannot stress enough the efforts that both local and Scottish Government have gone to in relation to securing the funding to meet this ask.

“Politicians and officers have worked tirelessly in partnership to review, re-prioritise, restructure and re-profile money, ensuring the impact on our communities is minimised.

“However, have no doubt, tough decisions have been taken and there will be delays to programmes and projects within communities to meet these pay demands.

“No new money has been identified for this offer – it is the ultimate example of ‘reduce, reuse and recycle’ in finance terms.

“Strikes are too damaging to our children and young people, their families and our communities across Scotland for us not to have taken this action.”

Keir Greenaway, GMB Scotland’s senior organiser in public services, said on the new offer: “Our members have absolutely no interest in last-minute deadline dramas. All they want is a fair pay offer.

“Cosla could have made such an offer months ago but instead, despite our best efforts, chose to drag the process out for no good reason.

“We will now look at the detail of this latest offer. If it is a significant improvement on the last one, we will take it to our members.

“If it is not, the strikes will go ahead as planned.”

Unite has been asked for comment.