FOUR people were confirmed dead last night after a massive explosion reduced a four-storey factory to rubble.

In a painstaking and dangerous operation, firefighters pulled at least seven workers from the remains of the Stockline Plastics factory in Glasgow.

Late last night emergency services continued to search in the wreckage for further survivors and bodies.

As the search moved to its second stage after 9pm, all firefighters left the scene to allow specially-trained sniffer dogs to search the area for any signs of life under the rubble. Experts from Grangemouth-based International Rescue, who specialise in earthquake disasters, were using equipment that included carbon dioxide detectors and specialist cameras.

Just before 11.30pm, there was a flurry of activity when workers reported that equipment had detected a noise. This came an hour after a senior police officer said that all dialogue with those trapped in the rubble had been lost.

Strathclyde Police confirmed last night that, in addition to the four dead, 41 people were injured, 17 of them seriously.

Three of the victims died at the scene and the fourth, a woman, died later in the city's Western Infirmary, making it Scotland's most serious industrial accident in a decade and the worst disaster in Glasgow since the Ibrox stairway collapse in 1971.

The cause of the blast was unknown, but health and safety experts are focusing their attention on the factory's industrial ovens. Police ruled out a gas leak as a possible cause.

Brian Sweeney, Strathclyde's firemaster, earlier likened the site to an earthquake scene and said the full rescue operation, which was being treated as a major incident, could take as long as 48 hours.

With the situation changing minute by minute, emergency services confirmed a number of people remained unaccounted for, although all those in vocal contact had been rescued.

Two, a man and a woman, had made contact by mobile phone. It is understood a group of around 10 members of the company's senior management were holding a meeting in the top floor of the building at the time of the blast. It is thought they were the ones buried beneath the rubble. Mr Sweeney said the rescue services were working ''flat out'' to finish the rescue operation and give any further survivors ''the best chance of survival''.

Earlier he described the condition of those trapped, adding: ''Some are injured, some are badly injured, some are in shock, some will be traumatised and so we're having as much decent dialogue as we can with them, trying to comfort and reassure them and offer support under what must be very traumatic circumstances.''

The Queen was shocked to learn about the blast and sent a message of sympathy to the lord provost of Glasgow.

Up to 200 firefighters were involved in the rescue, some using thermal imaging cameras and heat-seeking equipment.

An RAF Sea King rescue helicopter from Leconfield in east Yorkshire flew to Glasgow to ferry specialist dog teams and their handlers to the site.

Firefighters were forced to operate with extreme caution because the building's wreckage was dangerously unstable.

Late last night, as rescuers worked on under arc lights, the dog teams came into their own. They arrived from all over the UK: from the Trossachs mountain rescue team, from Cumbria, and by helicopter from Lincolnshire. They burrowed into the rubble to detect signs of life that may have been missed by the firefighters.

Neil Galbraith, a fire brigade chaplain, said: ''These International Rescue dogs will never give up. What you're seeing here is the fire brigade at its very best. All the services are working together and the families should know that we are trying as hard as we can.''

About 60 people were believed to be working in the factory when it exploded. The building collapsed in seconds. Witnesses described a huge explosion and screams. Blood-soaked survivors ran into the street as debris rained down.

Thomas Smith, 20, a local resident, said: ''I heard an incredible bang and when I looked outside the whole side of the building seemed to have disappeared. There was no fire of any kind, just an eerie silence and then the screams started.

''I could hear women and men screaming for their lives. It was horrible.''

Survivors suggested at least one of the four ovens in the factory may have exploded.

Experts speculated that the nature of the manufacture of plastics could have been the most likely cause of the blast.

Andrew Rankine, of Glasgow University, said the light solvents used could become highly explosive at low concentrations in the atmosphere.

Jack McConnell, the first minister, described the accident as ''tragic and terrible'' and said his thoughts were with the family and friends of the victims.