Bosses at a Stevenage school have admitted they "still don't know" what caused a major medical incident which left three people in hospital last week.

Police, fire and ambulance crews were called to Martins Wood Primary School in Mildmay Road just after 1pm on Wednesday, June 12, following reports of a number of children and adults becoming unwell.

One adult and two children were taken to Lister Hospital by ambulance with suspected carbon monoxide poisoning, while a further six adults and nine children were assessed at the scene.

Despite the school revealing that two previous gas leaks had taken place, bosses claimed hospital tests found no carbon monoxide in any patient's system, and that two checks by engineers found no trace of the potentially lethal gas.

READ MORE: 'Like a movie' - Martins Wood parent on chaotic scene after carbon monoxide leak

A week on from the incident, a cause is still yet to be established, but a statement from the school suggested that "people becoming anxious" could have led to children and staff seeking medical advice.

"We still don’t know what caused so many people to feel unwell," the statement read.

"The situation caused a number of people to become anxious and it is possible that this contributed to so many people seeking medical advice onsite.

"At present, and after extensive investigation, no identifiable cause has been found.

"The three people who went to hospital had no traces of carbon monoxide in their systems and advice by medics was that the three incidents were unrelated and caused by other health conditions.

"While this is incredibly unusual, we believe these medical emergencies were not linked to each other."