The cause of a fire following an explosion which ripped through a school construction site and forced children to flee their classrooms is still unknown.

Surrey Comet:

Aerial view: junior school had been doing a £5.3m new build credit: NPAS Redhill

Neighbours to St George's College Junior School, Weybridge, heard an explosion on Friday before thick clouds of black smoke started billowing from the school.

From Friday: VIDEO: Day off? No chance. Pupils at Weybridge school hit by explosive fire have been sent back to class

A Surrey Fire and Rescue spokesman said: "We are currently still carrying out a fire investigation to establish the cause of the incident."

Eight fire engines, 40 firefighters, an aerial ladder platform and a water carrier were sent to the Thames Street school.

Children at the school, boys and girls aged three to 11, were said to be unhurt and "accounted for" by police.

A Surrey Police spokesman said the investigation was being led by the fire service and the cause was not thought to be suspicious.

Councillor Andrew Davis, for Weybridge North, who was at the school quickly after the news broke, said although many people heard the explosion, if it was not for the thick black smoke pouring into the sky the majority might have thought it was the sound of a car backfiring.

He praised the emergency services and those who helped redirect traffic.

Coun Davis said the speed of social media and reporting on the fire must have been both a comfort and a concern for anxious parents until "we knew the little ones were safe".

The firm carrying out the construction project, Feltham Construction, said: "At this stage investigations are ongoing and we have no further comment."

IID Architects, who designed the £5.3m project, declined to comment on the fire.

Building work had been going on at the school and the fire was said to be at the construction site. New classrooms were due to be built by 2016 as part of a two year project.

No children of staff were said to be near the site during the fire.

The headteacher of the junior school, Anthony Hudson, said in a statement released on Friday, March 13: "While this has come as a great shock, I am very relieved that not a single person was hurt in any way and I am particularly grateful to my colleagues and the emergency services for dealing with the situation so professionally.

"All the children have returned to class and we very much look forward to seeing them again on Monday morning for business as usual."