A scheme designed to deal with predicted rising sea levels in a low lying area of East Devon could create malarial swamps, regularly flood a busy road, erode prime agricultural land, wreck buildings, ruin livelihoods and see water meadows disappear forever under acres of mud.

The claims are being made by members of a Topsham- based protest group who will be hoping to have their say tomorrow at a meeting being staged by the Environment Agency (EA) in Exeter.

The event has been arranged so experts from the EA can outline future options which they believe need to be adopted in the landscape surrounding the Lower Clyst River as it flows into the Exe.

But members of the Save the Clyst campaign have demanded a platform at the meeting. They believe the "managed realignment" being prescribed by experts from both the EA and the RSPB will have a disastrous effect on the landscape and ruin what is a productive swathe of farmland.

"We've already got mosquitoes which have moved into the marshes lower down," says campaign organiser Nigel Cheffers-Heard, adding that his wife Caroline who runs Topsham's Bridge Inn has been bitten by one.

"They are nasty," said Mr Cheffers-Heard. "A top entomologist comes into the pub and he tells us they are the species aedes detritus and aedes caspius which have come across from Europe – their life cycles involve tidal action, salty ground, brackish water, in fact, everything this project is trying to introduce!"

A mosquito invasion is just one worry in a long list drawn up by members of the campaign group which is concerned about a possible retreat in the area's tidal defences.

"We had a meeting attended by local farmers and landowners and it was just about unanimous that people do not want the Lower Clyst turned into salt marshes," said Mr Cheffers-Heard.

But an EA spokesman said plans in the area were far from "done and dusted".

"No decision will be taken without a full consultation with the landowners," said the spokesman. "The banks along the Lower Clyst were constructed many years ago as part of a scheme to increase agricultural production – maintaining the standard of protection they provide is not sustainable in the face of climate change.

"Increasing the height of banks would also increase water levels and increase the risk of flooding at Clyst St Mary.

"A partnership of the EA, Natural England, the RSPB and local authorities has been working with local farmers to investigate the possibility of creating wetland habitat by restoring tidal inundation of the floodplain."

Pulling back from hard- pushed sea-defences is called a "managed realignment" – and if such a policy were to be adopted, the resultant marshes would become a haven for wildlife such as wading birds.

The RSPB's public affairs officer Tony Whitehead said the partnership was, "exploring various ways to manage a difficult situation – how to deal with the inevitable changes caused by rising sea levels at a time when the taxpayer can no longer fund the maintenance of all flood- banks.