Image caption Online retailer Amazon employs more than 20,000 people across its eight warehouses during its peak Christmas season

A BBC investigation into a UK-based Amazon warehouse has found conditions that a stress expert said could cause "mental and physical illness".

Prof Michael Marmot was shown secret filming of night shifts involving up to 11 miles of walking - where an undercover worker was expected to collect orders every 33 seconds.

It comes as the company employs 15,000 extra staff to cater for Christmas.

Amazon said in a statement worker safety was its "number one priority".

Undercover reporter Adam Littler, 23, got an agency job at Amazon's Swansea warehouse. He took a hidden camera inside for BBC Panorama to record what happened on his shifts.

He was employed as a "picker", collecting orders from 800,000 sq ft of storage.

A handset told him what to collect and put on his trolley. It allotted him a set number of seconds to find each product and counted down. If he made a mistake the scanner beeped.

"We are machines, we are robots, we plug our scanner in, we're holding it, but we might as well be plugging it into ourselves", he said.

Image caption Adam Littler went undercover as a "picker" at Amazon's Swansea warehouse

"We don't think for ourselves, maybe they don't trust us to think for ourselves as human beings, I don't know."

Prof Marmot, one of Britain's leading experts on stress at work, said the working conditions at the warehouse are "all the bad stuff at once".

He said: "The characteristics of this type of job, the evidence shows increased risk of mental illness and physical illness."

"There are always going to be menial jobs, but we can make them better or worse. And it seems to me the demands of efficiency at the cost of individual's health and wellbeing - it's got to be balanced."

Amazon said official safety inspections had not raised any concerns and that an independent expert appointed by the company advised that the picking job is "similar to jobs in many other industries and does not increase the risk of mental and physical illness".

The scanner tracked Mr Littler's picking rate and sent his performance to managers. If it was too low, he was told he could face disciplinary action.

When Mr Littler worked night shifts his pay rose from the daily rate of £6.50 per hour to £8.25 per hour.

Image caption Amazon said the safety of its employees was its "number one priority"

After experiencing a ten-and-a-half-hour night shift, he said: "I managed to walk or hobble nearly 11 miles, just short of 11 miles last night. I'm absolutely shattered. My feet are the thing that are bothering me the most to be honest."

Amazon said new recruits are warned some positions are physically demanding and that some workers seek these positions as they enjoy the active nature of the work. The company said productivity targets are set objectively, based on previous performance levels achieved by the workforce.

Those on the night shift work a four-day week with an hour's break per shift.

Experts have told Panorama these ten-and-a-half-hour night shifts could breach the working time regulations because of the long hours and the strenuous nature of the work.

Barrister Giles Bedloe said: "If the work involves heavy physical and, or, mental strain then that night worker should not work more than eight hours in any 24-hour period.

But Amazon said its night shift is lawful. They said they sought expert advice to ensure the shifts "comply with all relevant legal requirements".

Amazon said it had invested £1bn in the UK and created 5,000 permanent jobs.

It added that it relied on the good judgement of thousands of employees. The company said: "Together we're working hard to make sure we're better tomorrow than we are today."

Panorama: The Truth Behind The Click, BBC One, Monday 25 November at 20:30 GMT and then available in the UK on the BBC iPlayer.