A multi-billion-pound barrage across the Severn estuary, backed by Peter Hain MP, could generate a huge economic boost for south east Wales.
Consortium Corlan Hafren wants to harness tidal power between south Wales and Weston-super-Mare to create electricity.
M Hain, 62, has quit as shadow Welsh Secretary to back this bid.
But environmentalists object over fears the barrage would impact salmon stocks and birds feeding on the mudflats.
Mr Hain said: "I want to make a different contribution.
"I want to take forward the vital project, the biggest investment ever in Wales, £30bn investment, for the Severn barrage, and also the biggest single source of renewable energy in Europe and one of the biggest in the world.
"That's what I want to do and you can't really do that as shadow secretary of state."
He added there were "all sorts of obstacles in the way" and that was one of the reasons behind him standing down.
"We will need to get a rather complex hybrid bill through Parliament that's got to be a private bill - it will need government support," he said.
The UK government previously rejected plans following a two-year feasibility study found the cost could be up to £34bn, double the initial estimate.
It did not, however, rule out private schemes.
So last year, Corlan Hafren submitted a business plan to the Department of Energy and Climate Change outlining a new proposal.
Mr Hain said this project would not call on public funds but would be entirely privately financed.
It wants to generate electricity on both ebb and flow tides and includes an extra 800 turbines.
The change may have some environmental benefits in reducing flood risk and improving coastal drainage upstream, according to the consortium.
But the barrage will still reduce the tidal range, which is the second highest in the world, from 14 metres at spring tide to seven.
Roger Falconer, professor of water engineering and hydrology at Cardiff University, carries out research for the consortium.
He said someone of Mr Hain's "considerable stature" would give the project "a big boost".
Prof Falconer previously involved in modelling the effects on tidal movements of the last scheme to build a barrage from Cardiff to Weston, which was proposed by the Severn Tidal Power Group (STPG).
"I have been working on this for 15 years and the more work I do on it the more convinced I am that the environmental issues are not as serious as many believe.
"The pros outweigh the cons," he added.
A spokesperson for the RSPB said it "wishes to see Severn tidal energy harnessed but not at the expense of important and protected wildlife".
"We will support technologies or alternative schemes that could deliver this, but the conventional barrage would be an environmental disaster and be in breach of the EU habitats directive."
A spokesperson for Friends of the Earth agreed harnessing tidal energy was "vitally important".
"But this project is the wrong solution - tidal energy can be captured by other technologies with considerably less damaging consequences," they added.
"Putting all our eggs into one risky project would block other possibilities, and the growth in jobs and technology these could produce."
The Cardiff to Weston barrage was expected to provide 5% of the UK's electricity and create thousands of jobs.