General union GMB said it had issued a formal written notice to the company that all members will take strike action between the hours of 5am Friday 19 February and 8am Saturday 20 February 2016.
During the dispute there will be picket lines at 18 main water and waste water treatment works, with GMB saying it expects approximately 700 staff to take part in the strike.
The strike is part of a long-running dispute over UU’s proposals to introduce changes to its defined benefit pension scheme.
Last month GMB announced that it was going ahead with an official strike ballot to ensure the company gives an undertaking to make no changes to the scheme without negotiations. The ballot closed on the 26 January with 82.1 per cent of members voting to take part in industrial action.
UU told Utility Week it was “disappointed that the GMB had decided to ask its members to walk out for a day”, but that it had “plans in place” to make sure its services to customers are not affected by action by “only a very small number of employees”.
A UU spokesman said: “We’ve been consulting with four unions on a proposal to close the defined benefit pension scheme affecting some of our employees. That proposal has now been taken off the table, at the request of the unions, until we know the extent of costs to continue to keep it going, which will be in May.
“This has been accepted by three of the unions which represent the majority of the people who would have been affected by this. We continue to want to resolve any differences by talking, as we have always done.”
However, GMB regional officer Eddie Palmer claimed that, while UU has now temporarily withdrawn the proposal, this has come with “a number of strings attached”.
“The company has made it clear that the offer of withdrawal is on the basis that the unions commit to talks which will include changes to the current pension scheme, up to and including closure,” he added. “When challenged further on this point the company confirmed that the withdrawal offer will be taken off the table in the event of the unions failing to commit to such talks.”