Firefighters from Mold, Buckley and Deeside were called to the Colomendy Outdoor Education Centre, Loggerheads, at about 1.30pm yesterday, after receiving reports that a large oak tree had fallen on to six propane gas tanks.

The tree smashed the isolation valve of one of the cylinders, causing gas to be released into the atmosphere.

Amid concerns about potential gas hazards, 110 children and staff from the education centre and people from nearby homes were evacuated.

The electricity supply to all surrounding properties was isolated as firefighters carried out checks for gas leaks, and the cylinders were deemed safe in the early evening, when gas company staff began emptying all six cylinders and removing the one damaged by the fallen tree.

Ian Williams, county operations manager for the fire service, said yesterday: "As the gas is heavier than air, it will follow the contours of the land. We used a main jet to disperse the cloud of gas to make the area as safe as possible."

A group of 59 children from London and Italy who had been staying at the centre, run by Kingswood, were sent to stay at the company's other site in Denbigh for the night.

Gas company staff are expected to be at the site until lunchtime and decisions were due to be made this morning on whether the centre can accept a new group of children, who were due to arrive at 3pm today.

Kingswood confirmed it would be contacting the two schools who were due to arrive today to advise them of what had happened.

A spokesman said the evacuation of the site, which is owned by Liverpool City Council, went smoothly and the repairs should be completed within the next 24 hours.

Colomendy was originally a centre for wartime evacuees who were sent from major cities, to give them an experience of the countryside.

It is now used by hundreds of schools as an outdoor education centre, where children spend a few days taking on new challenges.