Firefighters are investigating the cause of the blaze

Six fire crews and an aerial platform were called to Rhosnesni High School in Wrexham, just before 0600 BST.

North Wales Fire Service said the fire, which broke out in the hall, was being treated as a serious incident.

Lessons will not be affected as no pupils are being taught at the site, which is undergoing building work.

Worried neighbours alerted firefighters when the blaze broke out early on Wednesday.

Police and fire investigators have decided the construction workers can enter the school grounds and continue their work on one of Wrexham's new 'super' schools.

The scene will be combed for evidence of any wrong doing

North Wales Fire Service said the blaze was well alight when they arrived but they managed to confine it all to the hall.

Rhosnesni is one of three schools in Wrexham which are being merged into two "super schools".

Pupils from Rhosnesni, formerly known as St David's, are being taught at the Grove High School, which will shut once the reorganisation is complete.

The former St David's, the Grove and Bryn Offa School are closing and pupils will be taught at the rebuilt Rhosnesni and Ysgol Clywedog (the old Bryn Offa) sites.

Building work is being carried out at Rhosnesni to adapt the site and it was due to open fully in July.

The project has already seen costs rise from �12m to �22m.

It is not yet clear whether the incident will delay the building work, but Wrexham Council's education spokesman Terry Garner says they will try to get work completed by the summer.

"With the combined efforts of builders and the authority we hope it will stay on target," he said.

"We owe a lot to the speedy response of the fire service," he added.