COUNCILS and the Environment Agency are joining forces for a combined "robust" challenge to plans for a huge new reservoir in the county.

It is the latest move in the row over Thames Water's scheme for a ten square kilometre lake on farmland between Abingdon and Wantage.

They say the reservoir, containing 150 billion litres (33 billion gallons) of water and half the capactity of Lake Windermere, is needed to supply the growing population of the South East.

The deadline for views on the company's first stage consultation held in September and October expires on Thursday.

But at a meeting in Abingdon more than 60 council representatives pledged that work would continue right up to the proposed public inquiry expected in 2008 to "scrutinize and robustly challenge" the water company's case.

Joining forces are Oxfordshire County Council, the Vale of White Horse and South Oxfordshire District Councils and the Environment Agency along with parish councils whose areas are most likely to be affected by the reservoir proposed between Steventon and East Hanney. The link-up will share expertise and avoid duplication.

Terry Cox, the vice-chairman of the Vale council's Reservoir Advisory Group. He said: "People need to understand that we will be vigorous in this challenge and will not accept anything until we have had a chance to examine every assumption and every conclusion.

"It is important to share information with our colleagues in other councils and we have agreed to continue with further meetings throughout the various stages of Thames Water's consultation period."

The leader of Oxfordshire County Council, Keith Mitchell, said: "The county's position at the moment is one of challenge and not of outright opposition. Thames Water has quite a number of questions to answer about the need for a reservoir, its location and scale."

The combined team believes there are three fundamental questions arising from the first round of consultations -how much water is needed for the next 25 years, what is the best option for providing water and, if a reservoir is proved to be the best option, where should it be located.

Mr Mitchell added that if it was ultimately decided that a reservoir was the best option then the councils would have to start examining the design of it "to make sure we secure the best possible options for the community."

Part Two of the consultation process when Thames Water will give more technical details about the proposed reservoir is due to get underway in January.