GMB London, which represents some water workers, called on Thames Water and the Government to implement Victorian-era plans to move water from Welsh lakes to London and the south east to help combat drought.
It comes after Thames Water confirmed it would be putting in place a hosepipe ban for millions in the capital and surrounding areas from August 24.
Officials at the union said the plan was “win, win” and could be done by restoring canals in the Cotswolds and the Sapperton Canal Tunnel near Cirencester.
“Thames Water should accept the water being offered by United Utilities from Lake Vyrnwy and get it to the Thames via the restoration of the Cotswold canals and Sapperton tunnel,” said Mark Holland, the union’s regional organiser for the water industry.
“This plan was covered in the Thames Water 2019 draft plan for water supply for London in the 21st century but is not included in the current list of things Thames Water plan to do.
“Instead of this very workable plan, one of the things Thames Water is planning to rely on is the hope of consumers cutting daily consumption from 145 litres to 125 litres.”
The branch said it would be “common sense” and a “financially viable” approach that would have the added benefit of restoring disused canals.
It said it would also be easier than building a new reservoir at Abingdon, Oxfordshire, which was first proposed in 2006.
However, the proposal has not been endorsed by the union’s branch in Wales.
Tom Hoyles, GMB political officer, told ITV Wales: “Any proposals are currently at an exploratory phase and are not official union policy.
“GMB Wales believes discussions on this matter need far more detail as to how the infrastructure would be developed, how it would affect our workers on the ground and how the water would be paid for.
“Nothing comes for free in this world.”
Thames Water has been contacted for comment.