Detainees at immigration detention centres across the UK are working for as little as 1 an hour, carrying out work that could potentially save private contractors millions of pounds each year.

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Detention centres run by private companies including G4S, Serco, GEO and Mitie are paying asylum seekers £1 an hour to paint, clean, serve food and even buff the floors late at night.

A report by the group Corporate Watch has revealed the extent of the work being carried out by detainees at nine locations across the UK.

In one month over this year at Harmondsworth, one of the UK's largest detention centres, detainess worked a combined 16737 hours.

At a rate of £1 per hour for most work, the contractor Geo paid out just £17,218. If the work was outsourced at the national minimum wage, it would have had to fork out over £105,610 that month alone.

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In total an estimated £3m in labour costs could be saved by contractors who are using detainee labour.

The Home Office notes that detainees in immigration centres are exempt from the minimum wage and could opt out of work, but detainees who spoke to Channel 4 News claimed they were being exploited and felt they had little choice but to work for low pay.

We spoke to a detainee inside Harmondsworth, who said: "We're being kept in here, we're not allowed to work and make a living.

"To come in here and say, 'Oh yeah, here's a job, you get a pound an hour'. I mean that's a slap in a face. All the dirty jobs in here are done by the inmates."

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(Video - Tryness Ncube, asylum seeker, and Don Flynn, Migrants' Rights Network)

"Out there if you employ a cleaner you're paying at least minimum wage. So to get £1 an hour, Jeez, it's a big slap in the face. I mean there are people in here who send money to their families with the money they get from their job."

Tryness Ncube (video above) is an asylum seeker from Zimbabwe who spent time inside Yarl's Wood working as a cleaner. She told Channel 4 News she felt taken advantage of.

"It wasn't fair payment. UKBA (United Kingdom Border Agency) were taking advantage of the vulnerability of people knowing you would take the job because there is no other means of getting any income.

"I took the job because I wanted to call my solicitor. I spent everything I had from working on phone calls."

A Home Office spokesperson said: "The longstanding practice of offering paid work to detainees has been praised by Her Majesty's Inspectorate of Prisons as it helps to keep them occupied whilst their removal is being arranged.

"Whether or not they wish to participate is entirely up to the detainees themselves. This practice is not intended to substitute the work of trained staff."