A man critically injured in the Teesside power station blast died today to take the death toll to three.

Two workers were killed in the explosion yesterday afternoon and a fourth, who suffered more moderate burns, was stable in hospital today.

None of the victims in the blast, at the world's largest privately-owned combined gas heat and power plant, have been named.

The cause of the explosion was not known last night.

Inspectors from the Health and Safety Executive launched an immediate investigation at Teesside Power Station, at Wilton, between Middlesbrough and Redcar.

A blaze tore through the area following the blast and the station, which employs 200, was immediately shut down and evacuated.

A spokesman for plant owner, Enron Power Operations, said: "At about 2.40pm there was an explosion and subsequent fire.

"The explosion was in a transformer in the station's electricity generating hall."

About 30 firefighters found the dead and injured lying together at the scene of the explosion, and then tackled oil-fuelled fires throughout the building.

Shocked workers emerging from the plant talked of confusion after the explosion and said they had not even been told some of their colleagues had died.

Labourer Anthony Haley was on his first day at the plant when the blast happened.

The 21-year-old from Normanby, Middlesbrough, said: "I was working with some of the lads and we saw lots of smoke coming from round the corner.

"The next thing was people were saying, 'Get out and evacuate the place'."

Rumour circulating among workers suggested that a gas cylinder had fallen into a tank, causing the explosion, although Enron Power Operations chief executive Tim Underdown said it was too early to speculate about the cause.

He added: "Our immediate concerns are for the families, friends and colleagues of these employees and we have assembled a team to offer all possible support.

"The power stations remains off line and no decision will be taken for some time yet regarding its operation.

"Everyone at Enron is deeply shocked and distressed by this incident and I have spoken to the chief executive officer in the United States and he has immediately made arrangements to fly here."

Last night, Cleveland Police stressed there was no danger of further explosions or of risks to people in the surrounding area.

It took firefighters two hours to put out the fires. One crew remained at the scene into the night.

Redcar MP Vera Baird said: "It's dreadful. There must be a thorough inquiry."

Controversy over safety at what is Europe's biggest gas-fired power station dogged it even before a single watt was generated.

Safety standards came under fierce scrutiny after a series of accidents during its construction, one of which left a man dead in 1992 and sparked a mass walk-out among the 2,000 workers building the £850m plant.

A pipe fitter was seriously injured eight months later.

Three years ago, a court fined Enron and its maintenance contractor a total of £25,000 after a worker from Stockton received horrific burns and was unable to work again after a blast caused by a fuel leak.

Updated : 10:43, Thursday 9th Aug.