Southern Water has apologised to top girls' school Roedean for failing to tell it part of its grounds could become the site of a sewage treatment works.

The most expensive school in Britain, situated on a cliff top overlooking Brighton Marina, has suddenly learnt it could have a works treating all of Brighton and Hove's waste water, 95 million litres of sewage a day, in its grounds.

It has reacted with shock, especially after learning Southern Water could compulsory purchase part of the ground if councillors give the go-ahead.

Southern Water has apologised for not notifying Roedean, blaming Brighton and Hove City Council for not clarifying properly who owned the land adjacent to playing fields next to the Ovingdean roundabout.

Southern Water spokesman Geoff Loader said: "We have apologised to Roedean for not notifying it that Ovingdean was one of eight potential sites.

"We thought the land belonged to the city council, which initially gave us every indication it did. We wrote to the council on November 11 and only got a reply last Thursday, saying it did not own all of the land and the site we had identified belonged to Roedean School."

The school has been a top girl's school for daughters of the rich and famous for 118 years. Ex-pupils include Tory politician Baroness Chalker and actress Sarah Miles.

The school only discovered last Thursday that Southern Water had identified its grounds as a possible site.

Ovingdean is a feasible site because no buildings need to be demolished, there is easy access from the A259 South Coast Road and works could be hidden in a large hollow.

But Mr Loader, head of media relations at Southern Water, last night insisted: "There is no favoured site and we are carrying out a public consultation process with exhibitions between Shoreham and Newhaven."

Roedean school managers and the school's estates department met yesterday to decide its next move.

The school, in a statement, said: "We are still gathering information and cannot comment further at this stage."

School bursar Jeff Hynam, who lives in Ovingdean, told a packed meeting of residents at the weekend: "We rely on the intake of pupils for our income and having a sewage works next door could affect this."

No one from the city council was available for comment last night.