Police have rescued a British man who was likely trapped in a shed for 40 years and forced to work as a "slave", officials said.

The 58-year-old man was inside with only a chair and soiled bedding. His rescuers found him Wednesday during a raid at a residential site near Carlisle in Cumbria, United Kingdom, the BBC reported.

Authorities later arrested a 79-year-old man “on suspicion of modern slavery offenses,” saying he had held the man against his will for decades.

Gangmasters and Labour Abuse Authority [GLAA] officers discovered the trapped man after a call was made to a confidential helpline. The agency “works in partnership to protect vulnerable and exploited workers,” according to its website.

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Martin Plimmer, a senior investigating officer with the GLAA, said when rescuers found the man he appeared bewildered.

"When we found him he was like a rabbit in headlights and very confused,” Plimmer said, according to the BBC. "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 likely forced to do unpaid work from the age of 16 or 17, based on information from officers. It was not immediately clear if the man attempted to leave at any point.

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

Plimmer called the case “extremely sad and serious.”

"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,” he said.