Proposals to build a sewage plant on a greenfield site close to hundreds of homes have been described as "an anathema to planning".
The claim came on the second day of a public inquiry into the mooted £200 million treatment works in Peacehaven.
MP Des Turner said: "My understanding of planning law is that the use of such a greenfield site for such a development is completely out of court."
The five-week inquiry is hearing an appeal by Southern Water after East Sussex County Council failed to decide on its original application for a plant at Lower Hoddern Farm.
It will help Communities and Local Government Secretary Ruth Kelly and the Department for the Environment, Food and Rural Affairs (Defra) decide whether to approve the wastewater and sludge treatment complex.
Concerned residents have protested outside proceedings at the Meridian Centre about the smell, noise, traffic and visual impact the plant would have.
Leonard Sullivan, whose home in Southview Road, Peacehaven, backs on to the site, said: "Peacehaven will be destroyed. It's going to turn it into a slum and we'll lose the value of all our homes."
Quoting yesterday's edition of The Argus, where it was confirmed the site would smell, Mr Turner, MP for Brighton Kemptown, said he would not accuse Southern Water of "deliberately trying to smell out Peacehaven".
Instead he asked Damon Elliot, of Southern Water: "If you lived within 100ft of this plant, would you be content with it?"
Mr Elliot replied: "There is a need for this facility but I myself live quite close to a waste treatment works."
Mr Turner further questioned the decision to build at the greenfield site when three brownfield areas had been shortlisted for the plant - at Newhaven North Quay, Shoreham Harbour and the tip at Brookside Farm.
Mr Elliot admitted that neither of these three brownfield sites were "impossible" to use.
The company, however, chose Lower Hoddern Farm after a previous inquiry in 2002 rejected an application to expand its works in Portobello, Peacehaven, the inquiry heard.
Referring to this, Mr Turner said: "What kind of rigour is there in a process that disregards a clear steer from the planning inspectorate and that leads us to consider a site you would only consider out of sheer desperation?
"A complete anathema in planning terms, this site is also the worst in environmental terms and the worst in terms of impact to residents."
A new treatment plant is necessary to ensure the UK complies with the EU Wastewater Directive, put in place at the end of 2000.
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