“We’re no April Fools” – that was the message from scores of protesters who turned out to fight the expansion of a sewage works.

More than 60 residents descended on the Mogden plant in Isleworth over fears the approved expansion plans could mean unsavoury odours and mosquitoes would only get worse and house prices would plummet.

A number of protesters, from Hounslow and Richmond, carried placards reading: “We’re no April Fools” in reference to their belief Thames Water had not been honest over the expansion.

The protest was organised to coincide with a residents’ meeting held at Mogden by the company.

Thames Water reportedly claimed water and sewage regulators Ofwat would not allow them to cover all of the storm tanks, which is essential for odour control.

However, Isleworth councillor Phil Andrews said Ofwat’s permission would only be required if Thames Water was intending to raise the money for the coverings from the public by increasing water charges.

At Wednesday’s protest, members of Mogden Residents’ Action Group (Mrag) proposed the water company should spend its own money on covering the storm tanks, which store excess waste water until it can be treated.

Mrag member Marianne Walsh, from Crane Way, Whitton,said: “One of the problems is the terrible affect the expansion will cause on property prices. In this recession it’s a very big problem.

Everyone is going to know it’s been expanded and won’t want to buy. Whatever Thames Water say they are not living with it.”

Mrag member Steve Taylor said: “They [Thames Water] will profit by £400m but won’t cover the storm tanks. If they really wanted to do it they could pay for it themselves.”

Hounslow Council’s sustainable development committee voted to approve Thames Water’s application at the end of February, which will see the capacity of the site expand by 50 per cent.

However, residents from Isleworth and Hounslow fear the controversial plans will lead to an increase in unsavoury smells and a dramatic drop in house prices in the area.

Isleworth resident Salim Mughal, of Twickenham Road, said during the summer his family were often forced to lock themselves up in their house as the smell from Mogden was so bad.

He said: “Thames Water have been making so many promises but nothing is being done and when they extend it the pong is going to be worse.”

A spokeswoman from Thames Water said: "We are always willing to talk to residents about their concerns. We invited all the residents gathered outside the site to come inside and join the meeting where our experts were explaining the issues and answering questions, however they declined to do so.

"During the past two-years we have consulted widely with communities on the plans to improve the site. This included a week-long exhibition at Whitton and Isleworth libraries, and letters and leaflets sent to thousands of homes last year, including those in Whitton and St Margarets.

"The improvements will enable us to fully treat more of the existing flow into the treatment works, significantly reducing the frequency and the concentration of discharges of storm water into the tidal River Thames. This will improve the water quality of the river and will reduce the use of the storm tanks by 80 per cent.

"The potential for odour will also be reduced, as when the storm tanks need to be used, covered tanks will be used first. On the very rare occasions when uncovered tanks need to be used, a strict cleaning regime, agreed with the London Borough of Hounslow, will be in place to reduce any potential odour."