Four people, including a Shipley man, are being quizzed by detectives investigating threats and intimidation by animal rights' extremists.

The arrests were made at a number of addresses in the Humberside and West Yorkshire police areas.

It is understood all four people were arrested in connection with threats and intimidation of suppliers of goods and services to B&K Universal Ltd, a Hull-based company active in the field of laboratory animal science and biotechnology.

The intelligence-led operation involved officers from both forces working alongside other national police agencies, a Humberside Police spokesman said.

The Shipley man, aged 27, and a 17-year-old Leeds youth, were arrested in West Yorkshire, and two women, aged 30 and 56, were arrested near Hull in a pre-planned operation.

The arrests were made under section 145 of the Serious Organised Crime and Police Act 2005, which created a new offence of interference with contractual and similar relationships with the intention of harming an animal research organisation.

Detective Chief Inspector Christine Kelk, of the force's major incident team, said the intimidation was in the form of emails, phone calls and faxes, and some members of staff "have had personal visits."

She said: "It's part of a network of people who are intent on intimidating different companies who are carrying out their lawful business."

A large number of items were also seized during the searches, the force spokesman said.

All four people are in custody at police stations in Hull while inquiries and searches are concluded.