Specialist officers have rescued a potential modern slavery victim who is believed to have lived in a six-foot shed for 40 years.

An investigation is under way after the 58-year-old British man was found in the wooden structure, which contained just a chair and soiled bedding, at a residential site north of Carlisle in Cumbria.

A 79-year-old man was arrested on suspicion of modern slavery offences, while the victim was taken to be medically examined and assessed by specialist trauma officers.

Officers from the Gangmasters and Labour Abuse Authority (GLAA), which works to protect vulnerable and exploited workers, found the man in a dawn raid on Wednesday after a call to a confidential helpline.

“The information that was given to us was that he had been kept in the shed for a period of 40 years,” Martin Plimmer, GLAA senior investigating officer, told the BBC.

“When we found him he was like a rabbit in headlights and very confused … He was just in the clothes he stood up in and where he was sleeping in the shed there was just a soiled duvet on the floor. There was no heating and it was very cold. It was conditions that no human being should live in.”

Plimmer said the man was thought to have been forced to work without pay since the age of 16 or 17, and that it was unclear whether he had ever attempted to flee.

“He has been traumatised for such a length of time that it will be a slow process to win back his trust,” Plimmer said.

“In my long career I’ve never come across anyone who has been held as a slave potentially for 40 years and this, I think, could be the longest period of captivity that we have dealt with … It’s an extremely sad and serious case.”

In a statement, the GLAA added: “The operation has been supported by specialist officers from the National Crime Agency, the Cumbria Housing Department who are investigating possible health and safety breaches, and officers from Cumbria police.”

It said that modern slavery exists in a whole range of forms across the UK and called on the public to contact the agency if they suspect someone is being exploited or abused.

The number of suspected victims of modern slavery has increased from 13,000 in 2013 to 136,000 in 2018, with authorities recognising they are dealing with an “evolving threat”.