The controversial multi-million pound expansion of a sewage treatment works could still go ahead.

Southern Water has revealed it successfully lobbied to have the land it needs at Telscombe Cliffs, near Brighton, excluded from the South Downs national park boundaries.

The water company needs a sophisticated treatment works for the Brighton area so it can discharge cleaner sewage into the sea to meet European regulations.

But its plans for a £60 million expansion of the Portobello treatment works were rejected in March 2001 following a public protest and three-month public inquiry.

Telscombe Town Council and Lewes District Council are calling for the beach to be reinstated in the park boundaries.

If they fail, the rocky shore will lose its Area of Outstanding Natural Beauty (AONB) status, which would enable Southern Water to put in a revised planning application to build on the beach.

David Neighbour, a leading opponent of Portobello and a town, district and county councillor, said last night: "This is the first time Southern Water has revealed it was behind the decision to have Telscombe beach excluded.

"There is a strong belief Portobello is back on the agenda. We are going to have to watch them like a hawk."

Southern Water said in a statement last night it had not given up on building on the beach.

The company also disclosed it had lobbied against other wastewater treatment plants it operated being included in the boundaries.

The statement said: "Since the outcome of the Portobello public inquiry we have set up a project team with a brief to look across the whole of the area at every possible site to provide a modern wastewater treatment plant to serve Brighton and Hove. This exercise is continuing.

"We consider it extremely unlikely that the AONB boundary will alter but if it did, then that alteration would need to be taken account of in undertaking this exercise."

The main reason the expansion of Portobello was rejected at the inquiry was because it was in an area of outstanding beauty.

But if the beach remains excluded, Southern Water would be free to put in another application as the AONB status would be withdrawn.