Southern Water’s proposal for a £600m plant at Fawley to convert saltwater from the Solent into tap water during droughts has been formally opposed by the New Forest national park authority.

A separate petition against the plan, which the water company said it would enable it to extract less water from ecologically sensitive chalk streams such as the Test and the Itchen, has attracted several thousand signatures.

Residents objecting to the plant say that it is a “bizarre” and “expensive” solution for projected water shortages in southern England, with Britain’s total water supply forecast to drop by 7% by 2045.