Up to 50 construction workers had to be evacuated after fire broke out at the Guardian Media Group's headquarters project in central London.

The blaze began at the eighth floor of the £56m King's Place office building at 8.30 on Monday morning. A 200 m exclusion zone was set up as a precaution against the explosion of four gas canisters on the site.

One person was taken to hospital with burns.

It was not clear as Building went to press what had caused the blaze, which led to the evacuation of 1000 residents from the area.

King's Cross mainline station was closed on Monday and Tuesday.

Sir Robert McAlpine, the main contractor on the project, declined to comment.

A 200 m exclusion zone was set up
A 200 m exclusion zone was set up

The situation was described by the police and fire services as "extremely dangerous". A police officer stationed at one end of the exclusion zone said: "I'm nervous just standing here in case it blows - it would be catastrophic. The whole situation is very unstable."

The office of architect DEGW is situated near the site on York Way. Director Andrew Harrison was in the office while the fire was raging.

He said: "At about 8.30, there was an explosion and I looked out of the window and there were flames shooting out the concrete core on the building site. They were about 3 or 4 m high and there was black smoke everywhere."

The London Fire Brigade tested the gas cylinders on Tuesday afternoon to find out if they were cool enough to be removed. Nobody else was allowed access to the site.

A spokesperson from the Health and Safety Executive confirmed that no action could be taken until the site had been secured. "The site is not accessible for 48 hours. When it is, if the fire service works out that the cause of the fire was health and safety-related of course we will launch an investigation."