Fears of a terrorist attack sparked a major chemical scare which brought Kingston to a standstill on Wednesday afternoon.

A decontamination area was set up in the town centre, roads closed and the area evacuated and cordoned off after a postal worker opened a post box in Eden Street at 2.30 pm and discovered a pile of white powder inside.

Fearing what the substance might be, a major emergency situation was set in motion. Paul Massider, Station Commander at Kingston fire station, who attended the incident, said: "We had no idea what the powder was, but given situations which have occurred with white powder in the past we had no option but to take the fullest precautions. We always start from the worse case scenario and come down - then we are in front of the game."

The postal worker was taken to Kingston Hospital and a police scientific advisor kitted out in a full body protective suit with compressed air pumped into it, took samples of the powder. Firefighters also wearing protective suits stood by and a decontamination area was set up so that if the scientific officer did become contaminated he could be dealt with quickly.

Said Mr Massider: "We set up safe systems of working so that in the event of the officer collapsing, or anything more serious happening, we had a back up system. It was important to us that we did not put any members of the public or members of the emergency services at any unnecessary risk and to identify the powder."

The post box was sealed while the powder was identified.

Said Commander Massider: "We were quite happy that, whatever the powder was, it was contained and not blowing about."

While the emergency services in Kingston waited for the powder to be analysed, mass decontamination units started to be moved by road from Park Royal towards Kingston, in case the powder proved dangerous and large numbers of people needed to be decontaminated.

Almost three hours after the incident started the powder was identified as cornflour.

A spokesman for Kingston police said: "The white powder found in the post box was thought to be a noxious substance so it was treated as a chemical incident, but it turned out to be harmless."

A spokeswoman for Kingston Hospital said: "We can confirm that the London Ambulance Service brought an individual to Kingston Hospital after exposure to an unknown substance.

"The accident and emergency department was closed for a few hours whilst precautionary de-contamination procedures were followed. The substance has been identified as harmless."

Roads around Kingston were gridlocked for hours as traffic which had been diverted away from the incident area, met up with traffic which had left the Hampton Court Flower Show early when it closed due to bad weather.