An air raid siren sounded over south east London today as more than 60 firefighters battled a blaze at a chemical processing plant.

The alarm was sounded as a precaution after the fire at the buidling in Greenwich threatened to affect a unit containing hazardous chemicals on the same site.

The incident, which saw smoke fill the air around the unit on Peterboat Close, was brought under control just before 7am after 12 fire engines were sent to the scene in the early hours of the morning.

Station manager Laurie Kenny, of Silvertown fire station, said the warehouse on fire contained "swimming pool chemicals" but the siren was sounded to alert the public to "more dangerous chemicals" in a nearby building.

This building, one of only a few in London required to have a siren for safety reasons, was not affected by the fire but the alarm was sounded as a precaution.

He said: "The purpose of the air raid siren is that it says things are going on at the site. It is a precautionary siren because there was a possibility this would go wider than that area."

The site is owned by Brenntag, which describes itself on its website as a specialist in "bulk and packaged chemicals". One road was cordoned off near the site to make space for fire crews, while a nearby foot bridge was closed off.

The smoke from the building was not thought to be harmful, but fire crews advised residents in the area to keep their doors and windows closed as a precaution.

London Fire Brigade Assistant Commissioner Dany Cotton said: “Our firefighters worked extremely hard to contain the fire, which damaged around half of a big industrial unit.

"Heavy smoke spread throughout the area and as a precaution, we have asked people to keep their doors and windows closed.”

Residents in south east London described being awoken by the air raid siren in the early hours of the morning. One wrote on Twitter: "Have had no sleep all night, because of that fire in Greenwich was there really any need for the alarm to go off for 5 hours!!"

Another, Sarah Hemming, wrote: "Up from 3.30am to 6am with air raid siren going off and smell of acrid smoke. Chemical factory fire nearby. Grim."

A spokesman for Brenntag said: "Our emergency procedure was initiated, advice was provided to the fire brigade and local management attended the site.

"The fire was extinguished and the situation swiftly brought under control by the emergency services. No personnel were injured. The damage appears to be confined to one warehouse and a full investigation has begun."

