More than two-thirds of assembly members have signed a letter urging the prime minister to back plans for a £1.3bn tidal lagoon in Swansea Bay.
The cross-party letter says the project would make Wales and the UK a “global leader” in renewable energy.
In January, a review for UK ministers said the scheme would make a “strong contribution” to energy supplies.
The developer needs to agree with ministers how much subsidy it should receive for the energy generated.
Tidal Lagoon Power would also need a marine licence to be approved.
Review author, former UK energy minister Charles Hendry said the lagoon would add 30p per year to the energy costs of each UK household, over its 120-year lifetime.
There are hopes of developing a network of larger lagoons around the UK coast, harnessing power from the ebb and flow of the sea’s tides.
The letter to Theresa May is signed by 43 of the 60 AMs, including Labour’s Huw Irranca-Davies, Plaid Cymru’s Simon Thomas, Conservative Russell George and UKIP’s Mark Reckless.
It says: “Our collective interest as politicians is in a brighter future for Wales and though we may not always agree on how to get there, we are wholly united on this matter.
“We offer you our clear support for a positive decision at the earliest opportunity for the Swansea Bay Tidal Lagoon to proceed, and to be the pathfinder for future success for Wales and the UK as a global leader in tidal lagoon technology and renewable energy.”
The UK government has said it is focused on ensuring affordable, secure, low-carbon energy and will make its decision “in the best interests of the UK energy in the long term”.