Masked thieves stole diamond necklaces and jewellery worth £650,000 in a raid on Westonbirt School.

The six-strong gang broke into the girls' school where the annual South Cotswolds Antiques Fair was being held.

They are thought to have planned the heist by mingling with antique dealers at the fair last Saturday only to return between four and six am on Sunday to steal.

Wearing ski masks and balaclavas the robbers broke into a reading room on the ground floor of Westonbirt School, where the jewellery was kept. They set off the security alarms and smashed display cabinets protecting the high-quality pieces of jewellery.

Alerted guards tackled the thieves but the gang fought them off and they fled with their haul.

One guard who tried to stop them was hurt when the raiders threw pieces of antique furniture at him and sprayed him with a fire extinguisher.

Gloucestershire police say they escaped in three cars, one of them a silver estate car.

The gang was last seen driving on the A419 near Cirencester Town Football Club. Police spokesman Matthew Ford said: "An entire stand of pieces from elite London jeweller Anderson Jones, who specialises in Edwardian and Victorian jewellery was also stolen, at a value of around £300,000- 400,000."

The items are said to include high-quality silver and gold brooches as well as diamond-encrusted necklaces dating back hundreds of years.

The fair was organised by Coopers Antiques Fairs, of Burnham on Sea, Somerset, which also provided the security guards.

Spokeswoman Sue Ede said the raiders were ruthless and obviously knew what they wanted. She said: "They must have given the cabinets some wallop by which time our security team was on top of them. It was obviously planned but I am just glad nobody was hurt. We are all professionals. We are very proud of the way our security guards handled it."

They also stole antique furniture estimated to be worth several hundred pounds.

Mrs Ede said the theft would not stop the fair returning to Westonbirt School at Christmas.

The fair re-opened on Sunday after police and forensic teams checked the area.