Members of GMB Scotland and Unison have overwhelmingly backed industrial action after rejecting a 3.4 per cent pay offer.
The action could disrupt essential maintenance and emergency response services.
Almost 80 per cent of GMB Scotland members supported industrial action, with nearly 70 per cent backing strikes.
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The ballot had a 61 per cent turnout, with 67 per cent of members supporting strike action and 77 per cent backing industrial action short of strikes.
Claire Greer, GMB Scotland organiser, said: “Our members are again being forced to fight for a fair pay rise from a company where the top executives are taking home record bonus payments.
“The overwhelming support for industrial action is no surprise to our members and, given the failure to negotiate a fair offer, should come as no surprise to the company.”
The unions will meet to detail plans before giving Scottish Water two weeks’ notice of action if no agreement is reached.
This dispute comes 18 months after a rolling programme of industrial action, including strikes and overtime bans, disrupted emergency repairs, testing, and maintenance at Scottish Water.
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Unions had accused the publicly-owned company of behaving “like a rogue employer” for linking a pay offer to a restructuring of grades and salaries.
Unison has also announced a ban on overtime and said workers will no longer be on standby.
If Scottish Water does not return to the negotiating table to discuss an improved wage rise, Unison says there will be mass walkouts.
Scottish Water has offered employees 3.4 per cent or £1400, whichever is greater.
The union says this does not even begin to compensate workers for a decade of pay cuts and fails to recognise their value.
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Emma Phillips, Unison Scotland regional organiser, said: “No one wants to disrupt services.
“Scottish Water staff know the provision of clean, fresh water and dealing with sewage for homes and businesses is vital, but feel they have no choice.
“Their pay has not kept up with inflation over the past 10 years, and most staff get paid less than £31,000 per year.
“By contrast, senior managers ensure their own annual pay is measured in hundreds of thousands of pounds.
“What’s needed is a sensible solution that gives hard-working staff a pay rise that’s fair.”
Tricia McArthur, Unison Scottish Water branch secretary, said: “Scottish Water directors need to come out of their ivory towers and listen to their staff.
“Employees are sick of being taken for granted and underpaid, while their work is farmed out to private contractors, causing even greater expense for the public.
“Scottish Water is struggling to keep its workforce.
“It will do a whole lot better in recruiting and keeping them by coming back to the negotiating table with a decent offer.”
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A Scottish Water spokesperson said: “No-one benefits from industrial action. We would encourage the trade unions to rejoin negotiations so we can settle this matter amicably.
“Scottish Water remains committed to reaching an agreement with our unions that avoids industrial action.
“As a responsible employer, we want to make sure our people are paid fairly. That’s why we’ve offered a 3.4 per cent rise, significantly above inflation, with a guaranteed £1400 minimum increase. This means those in lower salary grades receive the highest percentage increase in Scottish Water.
“With previous improvements to pay and conditions, this offer means Scottish Water will be spending 17 per cent more on employee salaries than two years ago. This is in addition to reducing our company work week to 35 hours.”
