The UK Government has confirmed it is not prepared to back the Swansea Bay tidal lagoon project.
The company behind the scheme, Tidal Lagoon Power, wanted subsidies similar to those for new nuclear power to build the £1.3 billion scheme, consisting of a U-shaped sea wall with turbines in Swansea Bay.
The lagoon had been backed as a "pathfinder" project to develop the tidal technology by an independent review for the Government.
But Business Secretary Greg Clark said however "novel and appealing" the project did not meet the requirements for value for money.
He said the electricity generated by the lagoon programme by 2050 could cost up to £20bn more to produce compared to generating that same electricity through a mix of offshore wind and nuclear.
That would cost the average British household consumer up to an additional £700 between 2031 and 2050, he said.
The company behind the £1.3bn project say the lagoon would power approximately 155,000 homes in Wales each year.
It is thought it bring more than 2,000 jobs in construction and manufacturing and would attract 100,00 visitors to Swansea Bay annually.
Thirty five million pounds has already been privately spent on project development.
Tidal Lagoon Power said the British coastline, which has one of the highest tidal ranges in the world, is an 'untapped energy source with huge potential'.
But government were reported to be questioning the the project in terms of costs, long-term subsidies and the environmental impact.
The First Minister said he has previously written to the Energy Secretary Greg Clark urging him to give the project the green light and pledged £200m of investment. Reacting to the news, he described the decision as 'short-sighted.'
Criticism of the decision came from across the political parties.
Labour MP for Swansea East Carolyn Harris said to the Business Secretary he will "never understand the frustration and anger felt in my city today" and called for the Welsh Secretary Alun Cairns to resign.
In response to resignation calls, Welsh Secretary Alun Cairns MP insisted he is proud of his record and is "disappointed" with the decision.
Co-leader of the Green Party Caroline Lucas described it as a "day of Government-induced environmental disaster".