More than £5m was paid out in in consultancy fees over the planned Severn Estuary barrage, which was later shelved, it has been revealed.
Ministers have defended the work as "valuable" in case the scheme was given the go ahead in the future.
Labour have described it as "a tragic waste of money."
It was estimated a tidal-power barrage from Lavernock Point, near Cardiff, to Brean Down, Somerset, could have met up to 5% of the UK's electricity needs.
Plans for the 10-mile barrage across the Severn estuary were shelved last year amid concerns about the potential costs involved.
However the BBC has learned that even though the scheme was shelved more than £5m was paid out in consultancy fees.
One firm, Price Waterhouse Coopers, was paid £350,000 to examine financing and ownership.
The shadow Welsh secretary Peter Hain said the scheme would have created thousand of jobs and accused the coaltion government of "frittering away millions" on consultants.
A UK government spokesman said the work meant much more was now known about the costs, benefit and impact of Severn tidal power.
Before it was the shelved, the 10-mile (16km) barrage proposal, known as the Cardiff-Weston barrage, was one of five shortlisted schemes to harness renewable energy from the tides of the Severn Estuary, which has the second-largest tidal range in the world with 42ft (12.8m) tides.
The barrage would have harnessed water power using a hydro-electric dam, but would have been filled by the incoming tide rather than by water flowing downstream.