A 17-year-old boy has been arrested following the closure of Builth Wells High School due to a smoke bomb incident earlier today.

Dyfed-Powys Police and partners are currently dealing with the incident which saw students evacuated from the site during exams earlier due to a "suspected smoke/sulphur bomb".

Police say 20 people were affected by the substance, causing breathing difficulty and nausea to those exposed to it, although the substance is not believed to be dangerous.

A statement from the police issued this evening says: "A 17-year-old boy has been arrested, he remains in police custody and the investigation is ongoing. Police are treating this as an isolated incident which is not believed to be terrorist related at this time."

"A multi-agency response was activated and is ongoing. The primary focus of this response is to ensure the safety and welfare of all pupils and staff at the school, and members of the public in the surrounding area.

"Specially trained officers from all agencies attended the scene and conducted an assessment which has indicated the substance is not believed to be a danger."

The Welsh Ambulance Service confirmed the dispatch of two emergency ambulances, one rapid response vehicle with a Clinical Team Leader and National Inter-agency Liaison Officer, while hazardous substance teams have reportedly been despatched from Cardiff to attend the scene.