THREE letters containing white powder were opened in a mail sorting office, sparking a major security alert on Wednesday.

About 50 workers were evacuated from a ground-floor office of the credit reference agency Experien amid fears they had been exposed to a toxic substance. It was later confirmed to be non-toxic and the incident is being treated as a hoax.

The office is housed in the three-storey Reality building in Thynne Street, Bolton - the former home of Great Universal Stores (GUS).

The letters, believed to contain credit card applications, were opened by machine at 11.15am although four members of staff came into contact with the powder. They were immediately placed in isolated rooms and examined at the scene by paramedics as fire crews were called. The offices was sealed off.

Fire crews called for a specialist chemist who examined the substance and later confirmed it was harmless. It was being tested on Thursday.

A police spokesman added: "Initial tests on the substance have been carried out and the powder is believed to be non-toxic. No one was injured and staff from the premises made their way home later in the afternoon.

"Further tests are being carried out on the powder and enquiries are continuing."

It is the second time the office block has been evacuated due to a suspicious package. Last May, 500 employees were evacuated after staff opened a package containing white powder, pieces of paper and a written warning. Two workers who touched the powder developed a rash. It was later found to be a harmless hoax.

A spokesman for Reality said: "The results of the forensics on the incident came back showing the substance to be non-hazardous crystals of some form.

"We are now getting the site back into ordinary operations."