Environmental, not construction costs, are blamed

It found that costs has escalated and concludes that with future schemes of the same scale, estimates on final costs must be more realistic.

The audit committee's job is to keep a close eye on how public money is spent.

It has been known for some time that the original estimates for the landmark barrage - a 500-acre freshwater lake - had been exceeded by 15%.

Last summer, a report by Auditor General Sir John Bourn blamed the increase in costs on measures to improve water quality.

In 1995, the then Welsh Secretary, John Redwood, set a budget of £191m for the project

By 2000 year, the figure had increased to £213.4m.

The report also sets out unresolved issues and the environmental obligations linked to the project

Auditor General Sir John Bourn

And a report released to the assembly suggested that further water quality measures could push the budget up to £220m.

The report also highlighted the increase in estimated costs on the legal requirement to provide an area for the birds displaced by the building of the barrage.

The cost of providing a reserve on the nearby Gwent flatlands nearly doubled from £5.7m to £10.4m.

Cardiff County Council is now responsible for the running of the barrage and will receive funding from the assembly.

'Useful lessons'

But Sir John said it was important for the assembly to develop ways of helping to control the costs.

He added: "The report points to useful lessons for the management of projects of this kind.

"The report also sets out unresolved issues and the environmental obligations linked to the project.

"It will be essential for the assembly to ensure that these are dealt with in a way that minimises the risk of incurring significant liabilities and other substantial unplanned costs."