Brighton and Hove's sewage could be treated at a revolutionary treatment works under the sea inside a giant artificial island.

A Swansea-based firm wants to build the plant, which would sit on the seabed about 100m from the shore.

Kaiak has proposed Southern Water builds the plant, thought to be the first of its kind in the world, adjacent to the western arm of Brighton Marina.

The structure would be built using technology pioneered by the oil industry, before being assembled on site and sunk to the seabed.

Gerald Clark, of Kaiak, said housing the works inside an artificial island, expected to measure 100m by 70m, made more sense than building on the land.

A roof covering the sewage works, level with the top of the marina breakwaters, would be strong enough to be built on.

Mr Clark said: "The processes we are using are exactly the same as you find on land and the standard is exactly the same as the offshore oil industry. All we are doing is bringing the technologies together.

"It just has not been done before. I think this is a complete world first."

A wave of protests followed publication of eight possible onshore sites for Southern Water's new works to treat the city's sewage.

The shortlist includes Shoreham Harbour, Black Rock, Lower Sheepcote Valley, Higher Sheepcote Valley, Ovingdean South, Peacehaven East, and two sites at Newhaven.

Brighton and Hove will be unable to meet new European seawater rules unless the new plant is built, leading to the EU imposing heavy fines on the Government.

Mr Clark said his favoured location was at Brighton Marina but the semi-submerged plant could be built at a number of sites.

He said the sewage works would be fully enclosed and odourless.

Southern Water said it had been approached about the plant but it was too early to know whether it was a practical proposal.

A spokeswoman said: "Southern Water is considering it as part of its process in selecting a site. It is one of many options."