Staff represented by Unison and Unite will walk out on 22 and 23 April, following a one-day strike on 28 March.

Unison warned that emergency repairs and water quality checks will not be carried out during the action. Problems reported with water supply, sewage or drainage won’t be dealt with until the strike is over.

The union said the offer from Scottish Water – a pay increase this year of 2.6% or £1,050, whichever is higher – fails to compensate staff for a decade of real-term pay cuts.

Unison Scotland regional organiser Emma Phillips said: “No one wants strike action. We have done everything we can to persuade Scottish Water senior managers to put a fair offer on the table, but they are refusing to be reasonable.

“Staff have suffered a decade of pay deals which haven’t kept up with inflation. They’re fed up with being underpaid and Scottish Water senior managers must start to see sense.”

Unison Scottish Water branch secretary Tricia McArthur added: “Scottish Water workers deserve to be paid fairly for the services they provide.

“It’s important this vital publicly-owned service that so many rely on is protected, as are its dedicated workforce.”

Unite confirmed that around 500 of its members in key frontline roles will take part in the strike. Jobs impacted include roles in sewers, water treatment centres and on pipework. The industrial action will directly impact Scottish Water’s ability to respond to water leakages, flooding, pollution, and quality concerns.

The union has previously criticised Scottish Water executives for using talks through the conciliation service Acas as a means to deny workers a decent pay offer and to instead fall back on an inferior offer made last year. The “watered down” offer amounts to a basic pay rise of 3.4% or £1,050 for those on the lowest grades over a nine-month period.

Unite general secretary Sharon Graham said: “Our members at Scottish water provide a key function. Despite the essential work they do, they have seen their pay eroded for years, they are simply no longer prepared to tolerate this situation.

“Unite does what it says on the tin, and will support workers all the way in fighting for better jobs, pay.”

Sam Ritchie, Unite industrial officer, added: “Unite has now served industrial action notice to Scottish Water after the failure to reach agreement on pay. The action is likely to cause major disruption across Scotland, but the blame for that lies squarely with the executive team at Scottish Water.”

A Scottish Water spokesperson said: “We are very disappointed that we have still not been able to reach an agreement with the trade unions.

“We would encourage them to restart meaningful negotiations as soon as possible and to recognise the need to agree on a sustainable pay award for our people.

“No-one benefits from industrial action and our focus is on continuing to deliver for our millions of customers across Scotland.

“Our above-inflation pay offer is fair and progressive, prioritising the highest percentage increases in the business for those on the lowest salary grades – money that should be in employees’ pockets now.

“We have improved the offer in an effort to reach an agreement with the trade unions and we are now offering a combined deal for 2024/25 and 2025/26.”