OXFORDSHIRE County Council has torn apart Thames Water's case for building a reservoir the size of Heathrow Airport south of Abingdon.

The authority said it did not trust the company's forecast of a massive growth in population and water usage over coming decades and asked to see its working out.

It has also said it 'urgently' wanted to know much of the water from the 150 million litre artificial lake would be sold onto other water companies, as is now standard practice.

The list of demands come just days after the Campaign to Protect Rural England (CPRE) Oxfordshire branch announced Thames Water had already revised its population growth estimates after concerns were raised.

In a report dissecting the latest reservoir plan, published this year, the county council's director of planning Susan Halliwell warned the project would have 'substantial environmental, transport and landscape impacts in southern Oxfordshire, both during construction and when in operation'.

But digging into the company's figures, Ms Halliwell said she had 'significant concerns' over its population forecast, noting that in the box supposed to contain Oxford's forecast housing growth, Thames Water had simply put a question mark.

She told councillors: "The figures contained within this appendix underpin the entire plan and consequently there are significant concerns that not only has housing growth in Oxfordshire been miscalculated, identifying lower housing growth, but this may also be the case for other authorities within the Thames Valley."

She asked the council to recommend that Thames Water reassesses its predictions for Oxfordshire, 'and potentially elsewhere'.

The planning director also took the company to task over its notorious pipe leakage problem – currently estimated by the firm at 677m litres a day.

She warned that Oxfordshire County Council would expect Thames Water 'to speed up their programme of leakage reduction' to 'delay the need for a reservoir as long as possible'.

Finally, she challenged Thames Water over its assertion that the land between Marcham, East Hanney and Steventon was the only possible site for a reservoir in the entire South East.

Highlighting the fact that 'the scale of the new reservoir is driven by London’s growing demands', she said there was a 'lack of clarity on whether potential sites have been assessed across the South East region'.

She added: "If the reservoir is a ‘Regional Reservoir’ then the county needs to understand the process that has been undertaken to assess other sites."

Last week CPRE Oxfordshire said that it had helped persuade Thames Water to reduce its population growth forecasts by two million, pushing back the start date for the reservoir by years.

The campaign, along with the Group Against Abingdon Reservoir (GARD), challenged the firm's own calculations, and it then announced it would instead use Office of National Statistic projections, lowering the estimated 2100 population for the region from 16 million to 14 million.

As a result, Thames Water said the reservoir, originally planned to start operating in 2043, may not be needed until the 'late 2040s'.

However the CPRE's professor Richard Harding said its short-term population estimates were still 'way out of line'.

Thames Water spokeswoman Becky Trotman said: "When making our long term plans we’ll always use the best data available to us at that time and then, over the years, revise our plans as new data becomes available and this is exactly what we’ve done in the case of the proposed Abingdon Reservoir.

"Our long-term water resources plan, which includes the proposed reservoir, is currently out for consultation to help us gather comments and feedback and we welcome everyone’s opinion."

Thames Water is currently asking the public to comment on its draft Water Resources Management Plan by April 29.

The company says the South East is already 'seriously water stressed' and about a quarter of the water it puts into supply is lost through leaks.

Looking ahead, the company warned the population it serves is set to grow by two million by 2045, but climate change means the UK seeing hotter, drier summers and less rain.

By 2100, the company said it could face a shortfall of 864m litres a day.

Having looked at 'more than 200 options' to increase supply, it produced four 'preferred options', all of which include a new reservoir at Abingdon.

The proposals also include an 'ambition' to cut leakage by 15 per cent and encouraging households to use less water.

In August the firm will publish a report setting out comments received and how these have been taken into account in revising the plan, ahead of submitting it to the government.

The Secretary of State will then decide whether to approve the plan, make changes to it or hold a public inquiry into any aspects of the plan.

See thameswater.co.uk/sitecore/content/Your-Water-Future/Your-Water-Future