A thousand workers face an uncertain future after a drugs firm said animal rights activists might force it to leave Sussex.

Daniel Vasella, the chairman of Novartis, warned the company's multi-million-pound investment in the UK is at risk because of repeated threats to its scientists.

The Swiss research firm with £40 million headquarters in Wimblehurst Road, Horsham, is planning to focus its research funds at sites outside Europe.

Talking to reporters in China, where the company is considering major investment, Mr Vasella said: "The UK is the worst. It is scaring our people. If they become so scared that it becomes a major issue, we could come close to leaving."

The company invests £50 million a year on testing drugs and has 3,000 UK employees, including 500 in research and development and 200 scientists.

Mr Vasella said staff had suffered physical threats from people opposed to animal testing. Asked whether animals were used in testing at the Horsham site, a Novartis spokeswoman said: "Horsham is where our global unit for investigating respiratory diseases is based. The work done there conforms to the Government's standards and British law. Everything we do is what is required by law."

Sussex Enterprise's Steven Gauge said: "The local authority and the police need to take more action to protect businesses from harassment.

"People must take into account the effect their actions have. The loss of 1,000 jobs is not an insignificant dent."

Police said they had received six reports of anti-vivisection protests at the site since June.

The Horsham complex is home to PharmOPs, the company's pharmaceutical wing, employing 325. A further 675 work at three other Novartis units on the site, making tablets to combat respiratory diseases.