ANTHRAX hoaxers will be tracked down and prosecuted, Swindon's top police officer warned today.

Supt Julian Kirby, who heads the Wiltshire force's Swindon Division, made his pledge following the third scare in town. Around 500 staff were evacuated from NPower in Dorcan because a suspect package was found. Supt Kirby said hoaxers faced up to seven years in jail under new powers introduced by Home Secretary Jack Straw.

He also asked people not to panic and urged people to be calm and cautious if they received a suspicious package, rather than automatically assuming it to be terrorism-related.

He said that intelligence reports suggested that although Swindon was important economically, it did not have the high profile which made places such as London likelier targets for attack.

Supt Kirby said: "As well as causing inconvenience, chaos and cost, hoaxers, without a shadow of a doubt, cost lives. They take the emergency services, doctors and nurses away from their duties. As a consequence of that, we will investigate hoaxes thoroughly. Those that are identified will be taken to court for a local magistrate to deal with."

He said that people who receive a package should ask themselves if it is suspicious and why they should be a target before calling police.

He added: "Swindon houses the headquarters and premises of many large companies. However, its profile is not such that it would be high on international terrorists' target list."

The superintendent spoke after staff in the post room at NPower thought a parcel contained the deadly bacteria anthrax.

Fire engines, ambulan-ces and police cars were called to NPower in Dorcan, to deal with the emergency yesterday afternoon. Employees at the accounts and administration centre waited outside for two and a half hours before the all-clear was given.

Supt Kirby said an ambulance crew swabbed the hands of three staff who had been in contact with the mystery substance.

However, the substance itself, possibly a type of sand, was traced to a letter sent by a customer about a routine matter. Officers will now trace the customer in a bid to find out how and why the powder was in the letter.

NPower spokesman Alan Thomson said considerable disruption had been caused, but added that the firm believed the correct procedures should be followed.

This is the third time the threat of international bio-terrorism has affected workers in Swindon.

Managers at the Royal Mail sorting office in Dorcan decided not to evacuate the building while suspicious white powder was tested last Tuesday.

On Thursday, staff at the Book Club Associates warehouse at Groundwell Industrial Estate had to be decontaminated as a precaution during another scare.