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Backers of a £25bn plan for a River Severn tidal energy barrage say the project needs to get a green light in the next three years, or risk stalling.

The firm behind the scheme, Hafren Power, is in talks with UK government ministers and environmental groups.

Prof Brian Morgan, a member of Hafren's regional board, said any delays could risk it "going into the long grass".

The plan is for an 18km (11 mile) barrage between Cardiff and Weston-super-Mare.

A special Act of Parliament will have to be passed if the plans are to go ahead.

Hafren Power - whose backers include the company that built the London Eye, Marks Barfield, and the civil engineers Arup - hopes that process can begin next year.

Too long a delay and the company fears the opportunity may be lost.

"We do have a window of opportunity because we do have some support from the government, both in Wales and in Westminster," said Prof Morgan.

"There is a lot of opportunity in terms of accessing sovereign wealth funds.

"But the UK government will be facing an election in 2015, so we would need to put a Bill through parliament and get it approved before the end of this government, to make headway because then it will go into the long grass."

Neath MP former Welsh Secretary Peter Hain stood down from the Labour frontbench earlier this year to support Hafren Power's plans.

'Difficult negotiations'

He said the key to bringing a Bill forward and attracting investors is a commitment from the UK government to agree a guaranteed price for the energy the barrage would produce in its early years through special feed-in tariffs.

"The issue that needs to be tackled and resolved is the consumer price subsidy that all renewable projects get - wind, biomass, solar and so on - at the beginning of their life," he said.

"The Severn Barrage is no different. It needs a contract with an element of price subsidy for the first 25 years of its 100-plus year life, after which it is very cheap indeed.

"That's where the difficult negotiations need to happen, costings need to be bottomed out."

As well as proving the financial viability of its plans, Hafren Power also has to convince environmental groups of the merits of building such a large structure in the Severn Estuary.

The company says that it has improved on previous plans for a barrage that were rejected in 2010 by using smaller turbines that can generate power on both the rising and falling tide and at slower speeds.

The changes, the company says, mean its plans are more fish-friendly and will also reduce the amount of inter-tidal mud flats that will be lost to feeding birds, while still generating 5% of the UK's energy needs or enough to power 3.4 million homes.

Mark Lloyd, chief executive of the Angling Trust, however, is yet to be convinced.

"We have to say we're opposed to the barrage because we haven't seen evidence that says that it won't damage fish and wildlife and the geomorphology of the estuary. So we have to take a precautionary approach," he said.

"We don't want to see anything that damages that ecology.

Generating power

"If it can be shown that it won't have an impact or that the impact is reasonable given the scale of the energy that can be generated, then we might support it.

"But at the moment we don't have that evidence."

The Royal Society for the Protection of Birds, which recently met Hafren Power together with the Angling Trust and other green groups, is also reserving its judgment until it is able to see and assess detailed engineering plans for the barrage.

With a barrage at least a decade away from generating power, other environmental groups question whether spending so much money on such a long-term project is the best way to address climate change and rising carbon emissions.

David Fitzpatrick, chief executive of environmental charity Cynnal Cymru, Sustain Wales, said: "There are loads of people out of work, but there are plenty of jobs to be done.

"Let's look at preventative measures. Let's look at bringing in better awareness of what you do about insulation and then enabling that insulation to happen."

"We really should be thinking about not producing the energy in the first place, not trying to find something that could be a white elephant."