Fire services, police and paramedics were called to the Mundells, an area near Welwyn Garden City, Hertfordshire, shortly after 3pm.
Six wounded men were taken to the Queen Elizabeth II Hospital in Howlands, Herts. where one remains in a critical condition and one is seriously injured.
Following a subsequent search of the building a body was discovered.
A Hertfordshire Police spokesman said: "I can now confirm that one man has tragically died as a result of the incident.
"His family has now been made aware and a trained officer is providing them with some support at this time."
It is believed the blast happened after an argon cylinder exploded inside a building still under construction.
Following the incident around 150 builders were evacuated from the site and all surrounding roads were closed.
Jon Smith, Group Commander of Hertfordshire Fire and Rescue, said that the explosion had caused extensive damage to the inside of the building.
He said: "A cylinder of gas depressurised, there was no actual fire, the gas is inert, which acted like a missile and set off the other cylinders.
"Each cylinder weighs about 50kg and is about 5ft tall, they've made a real mess of the inside of the building, they've taken down walls and floors."
The argon cylinders were being used as a fire suppressant in the building, which is being built by construction firm Laing O'Rourke.
He said that the external structure was intact but that 50% of the internal structure had been affected.
The Health and Safety Executive are aware of the incident and are monitoring the situation.
A spokesman said: "The operation at the scene has now reduced and the area is returning back to normality.
"This includes the local road network which is operating as normally as possible."
They have advised workers that they should be able to return to work.
by Charlotte Bailey, 5th November 2008 11:14pm
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