Scotland faces a wave of bin strikes this summer after unions rejected a new pay offer, with the Edinburgh fringe at risk of a repeat of 2022 when mountains of refuse bags littered the streets.

The GMB and Unite unions said on Monday that their members had voted against a revised pay deal, which Scotland’s local authorities had described as “strong, fair and credible”.

Scotland’s major cities are likely to endure overflowing piles of uncollected refuse on pavements and at recycling points in August, mirroring scenes in Edinburgh during the festival two years ago.

Graham McNab of Unite urged the Scottish government to intervene. “A stinking Scottish summer looms unless Cosla [the umbrella body for local councils] and the Scottish government quickly sort this out by injecting more cash into a new offer,” he said.

Keir Greenaway, an organiser for GMB Scotland, said the proposal from Cosla of a 3.2% pay rise backdated to April was lower than that offered to English council workers. It is also less than cost of living increases suggested by the retail prices index, which is higher than the 2% headline rate of inflation. Cosla had originally offered 2.2% backdated to April and a further 2% from October.

“For Cosla to suggest this is a revised and improved offer only insults the intelligence of our members,” Greenaway said. “It is merely a repackaging of the previous offer already rejected as too low and too late. It begs the question why more weeks have been squandered on this when time is running out to halt imminent industrial action?”

Cosla, which has struggled with repeated real-terms cuts to council funding from the Scottish government, has told the unions its offer is the most it can afford. “We have been clear in all discussions with trade unions that the offer is also at the very limit of affordability for councils given the challenging financial situation we are facing,” said Cllr Katie Hagmann, Cosla’s resources spokesperson.

Unison was the first union to announce its members had rejected the offer, on Friday.

Unite said English council workers earning about £25,000 a year had been offered a rise equivalent to 67p an hour, while in Scotland the offer was equivalent to 41p an hour.

So far, unions in 20 of Scotland’s 32 council areas have mandated strike action, including in Edinburgh, Glasgow, Inverness, Stirling and Dundee, with branches in five more being reballoted.