The headquarters of the country's biggest animal charity is being offered for sale with an asking price of more than £3.5 million.

The RSPCA expects a developer to snap up the prime 1.92-acre site in The Causeway, Horsham. The site has outline planning permission for 33 homes.

Phil Heckles, of estate agency John Lytton in Shipley, Horsham, said it had only just gone on the market and no offers had been received.

The charity is to move after outgrowing its base, which was designed to accommodate 150 people and now houses more than 300.

Workers will move in August to a new £16 million building three miles away in Southwater.

Director-general Peter Davies said: "This is a very exciting move for the RSPCA. It is bigger and busier than ever before and demands on our resources continue to grow at a staggering rate."