A former Anglian Water engineer has cast doubt over whether drainage systems can withstand the increased loads created by a major new housing development.
Company bosses say they are working with council officials to draw up proposals for an infrastructure upgrade relating to schemes in the Marsh Lane and Lynnsport areas.
But campaigners against the project claim it would increase flood risk and harm the local environment.
And Alan Whitehouse, who worked for the water company in Lynn for 25 years, says he fears the scheme will increase the level of pollution which flows into the River Great Ouse.
He spoke out after attending a public consultation session on initial proposals for two parts of the scheme, known as Lynnsport 4 and 5, on Thursday.
Mr Whitehouse said officials told him the plans had been discussed with both Anglian Water and the Environment Agency, neither of whom had raised an objection to them.
But he is concerned about the additional loads that would run into the Gaywood Outfall sewer from the sites.
Mr Whitehouse, who left the water company in the 1990s, said: “It’s still the same size it was when I was there. There were problems with it all the time I was there.
“During rainy conditions, the sewer overflows. That discharge goes into a relief tunnel that pumps into the River Great Ouse. This is going to increase those flows. Nothing has been said about the implications. ”
A West Norfolk Council spokesman said there were no areas of combined sewerage within the development site itself, though there are still some in other parts of the town.
But Anglian Water has admitted an upgrade of the existing infrastructure will be needed to accommodate the level of housing proposed.
A spokesman said the situation in West Norfolk was being reflected across the region.
He added: “We are working with the developer to design a new system for the area.”
The project does include the provision of a new pumping station, for which plans were initially approved by the borough council in December 2014.
The council also said most responses to the latest plans were positive, with the provision for a link path to the King Edward VII Academy being particularly warmly welcomed.
The spokesman added: “There were some suggestions for improvements, including the provision of benches or seating, where possible.
“People were genuinely interested, asked lots of questions and seemed satisfied with the information they were given.”