The Environment Agency has raised serious doubts about a controversial plan to take millions of litres of water from the Thames and replace it with treated effluent.

The government's official watchdog says it has 'a number of reservations' about the scheme, which has 'not yet been shown to be feasible or environmentally acceptable'.

As a result, it has ordered Thames Water to develop alternative options to the scheme, which is designed to increase the water it captures and treats for millions of homes and businesses.

The concerns have been raised by the Environment Agency (EA) in a wide-ranging report, which is also highly critical of other proposals included in a Thames Water plan to ensure it can maintain supplies in future decades.

It warns that Thames Water plans to tackle massive leaks from its pipes do not go far enough.

And the watchdog also accuses the company of 'mishandling' the use of a £250m desalination plant in east London, which has been shut down for years, and even suggested it might have to be permanently shut down.

The EA says the company must carry out more work to assess the impact of plans for a vast new reservoir in the Oxfordshire countryside, south west of Abingdon, and it raises significant doubts over plans to transfer water from the River Severn to the Thames to supply homes in the capital and beyond.

Teddington

The Thames Water scheme involves taking up to 75 million litres of water a day from the Thames around 300m upstream of Teddington Lock and transporting it via existing tunnels under the capital to the east of London to supply customers.

Graphic shows water abstraction point on banks of Thames. Credit: Thames Water.

This water would be replaced with treated effluent from the Mogden sewage works via a new 15km pipeline that would discharge into the Thames closer to Teddington Lock.

The scheme has drawn thousands of objections from residents, local councillors and the borough's two MPs, Munira Wilson and Sarah Olney.

A first version of the scheme was blocked by the EA five years ago amid concerns about the impact on river life and fish related to changes in the chemical make-up of the water and its warmer temperature.

How the Teddington scheme would operate. Credit: Thames Water.

The Agency said Thames Water has brought the scheme back with a number of substantial improvements. However, it concludes: "The Environment Agency still has a number of reservations based on impact on the environment and viability in the long term."

The company has been given until August to provide more evidence that the Teddington scheme will not damage the environment – and produce alternatives.

Significantly, it states: "Given that the Teddington Direct River Abstraction has not yet been shown to be feasible or environmentally acceptable, the company should ensure alternatives are progressed.

"Thames Water should ensure any options selected are resilient, reliable and do not cause any adverse environmental impacts."

Leaks

Thames Water has a shocking record on leakage of treated water from its pipes amounting to an astonishing 635 million litres every day – a quarter of all supplies. Critics argue it should be doing much more to tackle this before taking water from the Thames.

Borough's streets hit by series of Thames Water main leaks.

The EA says the company has a plan to reduce leakage by 50% by 2050, but it says it may need to do more to ensure the taps do not run dry.

It states: "Thames Water leaks more water than any other company. The company has struggled to maintain its planned level of leakage, especially over the past year.

"Given the size of the leakage issues faced by Thames Water, we expect the company to invest in new research and development to identify ways it could substantially reduce leakage further than the 50% reduction target by 2050."

Desalination plant

The company has spent £250m of customers' money on a desalination plant at Becton, which can treat salt water from the tideway and make it suitable for the taps. However, it has not been used for years and the EA said the company needs to make a decision whether to use it or shut it down.

It was promised that the Gateway Desalination plant would supply around 75 million litres of water a day at periods of high demand - enough water for 400,000 homes and 900,000 people. However, it is expensive to treat the water and has effectively been mothballed.

The unused £250m desalination plant in Becton. Credit: Thames Water.

The EA said: "A number of other water companies plan to use desalinated water for future water supplies. Thames Water already has this ability, however the company has not managed the asset well and this vital resource has not been working for many years.

"The company must either commit to improving the asset to ensure it is reliable for regular operation or decide to decommission and select a new option.

"The company cannot continue to mishandle this asset when it provides crucial resilience to London in a drought."

The Agency said it is time for the company to demonstrate it can reliably provide the promised water 'or outline if the company will decommission the plant'.

Oxfordshire reservoir

The South East Strategic Resource Option (SESRO) is a reservoir planned to be built in Oxfordshire to provide water from 2040. It would provide water to Thames Water, Affinity Water and Southern Water customers.

What the vast new Oxfordshire reservoir could look like. Credit: GARD (Group Against Reservoir Development).

An initial plan suggests it would hold 100 million cubic metres of water, however there are suggestions it could be 50% larger at 150 million cubic metres. This would cover an area of 700 hectares - around 7 sq km (4.5 sq miles) – which equates to around 1,300 football pitches, making it bigger than the sprawling Gatwick airport or Gibraltar.

The EA says: "Thames Water's plan indicates that a larger reservoir (150Mm3) performs better on some metrics and could also offer additional resilience and environmental benefits.

"If the company's preferred solution is a reservoir, it should ensure that it provides the best value solution for its customers and the environment. The Environment Agency acknowledges this would cause disruption to local residents in the area during its construction and we expect Thames Water to manage these issues appropriately."

Transfer from River Severn to Thames

This idea has been floated since the 1970s and would transfer water from the North West and Midlands to the South East for use during a drought.

The water would be moved either by a new pipeline or by a combination of a new pipeline and restoring the Cotswold canals.

The EA says it is concerned the Severn may not have enough water in it at times of drought to then divert large quantities to the South East.

It states: "The Environment Agency is not convinced this is a viable solution. We have a number of concerns about its resilience, particularly in a

changing climate.

"Given that the River Severn to Thames transfer has not yet been shown to be feasible or environmentally acceptable, the company should ensure alternatives are progressed."

An Environment Agency spokesperson said: 'Climate change and population growth mean we need to take action now to ensure resilient water supplies are available to meet the needs of all users in the future.

'We require further evidence that the direct river abstraction proposal put forward by Thames Water would not have adverse impacts on the environment, along with proof of its long-term viability in light of climate change.

'We urge Thames Water to produce this evidence as quickly as possible and develop alternative options to ensure that it can deliver a secure supply of water for its customers for the next 25 years and beyond.'