An undercover TV investigator who filmed staff at a Medway youth jail allegedly abusing and mistreating inmates has denied he had been instructed to look for any wrong-doing.

Robert Padmore said his mandate was “to go in and investigate, to see what was going on and if there was something that wasn’t right, to film it”.

“It wasn’t to create a film-making opportunity,” he told a jury.

“It was just to film what was going on. I couldn’t film a child or member of staff for no reason. That would be like an invasion of privacy, so I had a switch.

“If I thought something was happening or going to happen I would turn it on and, if not, I would turn it back off.”

Mr Padmore was giving evidence at the trial of four G4S staff members - team leaders Anthony Dance, Gareth Evans, Matthew Cunningham and duty operations manager Christopher Lomax - who are all accused of misconduct in judicial or public office.

Dance, of Brookland Terrace, Lower Horsebridge, Hailsham, East Sussex, Lomax, of Timbertops, Chatham, Evans, of Tintagel Gardens, Rochester, and Cunningham, of Megby Close, Gillingham, all deny the charge.

Mr Padmore admitted he had no journalistic experience other than completing a course at film school.

He also conceded he had no previous experience of working undercover or with youths or offenders.

The investigation at Medway Secure Training Centre (MSTC) in Rochester for the BBC Panorama current affairs programme was his first assignment as a freelance journalist, having only completed a film course at the City of Islington College in London.

He explained that his lack of experience in the field was “necessary criteria” required by the BBC for the role as when applying for a job as a care officer at the centre for young offenders he would not have to disclose any reporting background. He was what was called in the media industry a “clean skin”.

During his 39 shifts between October and December 2015, he filmed Dance, 27, Cunningham, 37, Evans, 27, and Lomax, 36, allegedly being physically and verbally aggressive to “trainees”, aged 14 to 17.

The violence was said to have included headlocks, kicks, abusive language and threats to “annihilate, suppress or smash” inmates.

Mr Padmore agreed with Emma Goodall, for Dance, that filming was “partial and selective” and edited for the half hour programme.

Some of the 76 youths at the centre were considered to be the most dangerous and challenging within the criminal justice system.

Mr Padmore described his first shift on in October 2015 as “a baptism of fire”, having to intervene twice in fights between trainees and to make two restraints.

One happened after one youth stabbed another inmate in the neck with a pencil.

By his fifth shift, he said, he felt he was effectively running the unit.

Violence between inmates and attacks on male and female staff occurred constantly. It included trainees using pool cues and balls as weapons, or making “sharpies” in design and technology classes.

The investigator said serious injuries were caused. He was assaulted 10 times and once needed hospital treatment.

There were also problems with sexualised behaviour and assaults by youths on female staff.

Mr Padmore said it was a very demanding job both physically and mentally, and agreed with Miss Goodall that staff needed “decompression” to let their “mouth off” about inmates to colleagues.

The trial continues.