A 76-year-old man has been arrested on suspicion of administering poison after children fell ill at a summer camp in Leicestershire.

Police said an investigation was under way after eight children at the camp at Stathern Lodge were taken to hospital on Sunday. All have since been discharged.

Officers had gone to the camp, in Canal Lane, with East Midlands Ambulance Service after receiving reports about the children feeling unwell.

The arrested man was detained on suspicion of administering poison or a noxious thing with intent to injure, aggrieve or annoy. He remains in custody.

The police watchdog, the Independent Office for Police Conduct (IOPC), said it was investigating Leicestershire Police’s handling of the incident.

Police said once emergency services had arrived at the scene, a triage centre was set up at Plungar Village Hall to assess all children present at the camp.

Following an initial assessment, eight children were taken to hospital as a precaution and all have since returned home.

The force said the investigation was now being led by the East Midlands Special Operations Unit’s major incident team.

Officers remain in Stathern, in the Melton district of Leicestershire and roughly eight miles from Melton Mowbray.

They are carrying out further inquiries and providing reassurance to families affected.

Meanwhile, an IOPC spokesperson said it received a “conduct referral” on Tuesday from the force “relating to their handling of concerns passed to them over the wellbeing of a group of children”.

“Our assessment team has examined all available evidence and concluded the matter should be independently investigated by the IOPC,” they said.

“The investigation will look at whether there were any breaches of professional behaviour – namely a failure to carry out duties and responsibilities – that resulted in a delay in Leicestershire Police’s response to what was later declared a critical incident.”

Det Insp Neil Holden, the senior investigating officer, added: “We understand the concern this incident will have caused to parents, guardians and the surrounding community.

“We are in contact with the parents and guardians of all children concerned.

“Please be reassured that we have several dedicated resources deployed and are working with partner agencies including children’s services to ensure full safeguarding is provided to the children involved.

“This is a complex and sensitive investigation and we will continue to provide updates to both parents and guardians and the public as and when we can.”

At a press conference on Thursday afternoon, Assistant Chief Constable James Avery took questions from the media.

The first, from the BBC, asked what substance the children came into contact with.

He replied that, other than the fact that they came down with symptoms of feeling unwell, the police could not elaborate further for now.

He also refused to comment on the ages or gender of the children involved.

ACC Avery finished by reiterating that the care and ongoing welfare of the children involved is of the utmost importance.

A dedicated major incident public portal has been set up, and anyone with any information is asked to report it online here.

Local MP Ed Argar, Conservative member for Melton and Syston, described the reports as “clearly shocking” and “deeply concerning”.

“Our thoughts are with the young people affected, wishing them a full recovery,” he said.

“While, understandably, people will have a lot of questions about what has happened, it is important we remember this is a live police criminal investigation, so I would caution against people speculating while the police do that work.

“I would, however, encourage the police to provide as much information publicly as they can, as soon as they can, but without prejudicing that vital investigation.”

Isaac Ashe, a BBC reporter at the scene, said aside from a nearby hospice charity shop and coffee spot, there is “not a lot else but fields and trees here to see”.

Phil Dewey, 75, who was walking his dog in the normally quiet area, said he was “quite surprised to find police cars and an incident going on. It seems very bizarre”.

Elvas Madanombe, from the Friday Apostles church in Sheffield – which has regularly visited Stathern Lodge in the past – described the news as “horrific”.

He added it was “unimaginable” and said the group would have to re-evaluate visiting the lodge next week.