Workers at Northumbrian Water will start working to rule today, and have called a strike next week in a dispute over plans to close their final salary pension scheme.
GMB and Unite members will stop doing any overtime from 16:30 today and Unite members have voted for a 24-hour strike on Monday.
The unions have been locked in a dispute with the company since January to plans to replace the final salary scheme with Career Average Revalued Earnings (CARE) arrangements.
Unite regional officer Pat McCourt said: "When Li Ka-Shing took over Northumbrian Water in 2011, there was a pledge that all existing commitments would be honoured.
"Sadly, this promise has been broken as the company now wants to introduce a career average scheme from 1 January which may mean our members facing pensioner poverty, as they could lose tens of thousands of pounds in their retirement years."
Unite balloted 100 members for strike action and work to rule with 61% voting in favour of strike action and 80% voting for industrial action short of a strike.
GMB balloted nearly 600 members for action short of a strike with 77% voting in favour. It is reviewing whether it will join Unite in striking.
"We are starting with this and then we are going to have a look at it," said GMB regional officer Maxine Bartholomew.
Northumbrian Water's customer director Claire Sharp said:
"We expect the majority of our 3000 employees to continue working normally. Our priority will of course be to safeguard customer service and protect the environment and we have well planned resilience preparations to continue to do so. Consequently we do not expect action to impact greatly on our service to customers.
"We regret that in response to changes to our final salary pension scheme some of our union members have decided to take this industrial action.
"We are in the minority of companies being able to keep a defined benefits scheme open and we will still be providing our employees with one of the best schemes in the water industry."
GMB is also locked in a dispute with United Utilities which it says has revealed plans to close its defined benefit scheme to future accrual
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