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US presidential candidate Michael Bloomberg has said that his campaign "unknowingly" used prison workers to call voters on his behalf.

The billionaire Democrat said he had cut ties with a vendor which operates two call centres in state jails.

Mr Bloomberg said he learned about the company's use of prison labour only after receiving a call from a reporter.

He entered the election race in November with one of the largest-ever political ad buys.

His campaign's use of prison labour was first reported by news site The Intercept, and confirmed by Mr Bloomberg in a statement on Tuesday.

"We do not support this practice and we are making sure our vendors more properly vet their subcontractors moving forward," said the former New York mayor.

'We didn't know'

According to The Intercept, Mr Bloomberg - using a third-party vendor - hired call centre company ProCom to make phone calls for his campaign.

The firm operates two centres in Oklahoma state prisons. An unidentified source told The Intercept that incarcerated people were contracted to make calls from at least one of these prisons - a minimum-security women's prison with a capacity of over 900 inmates.

In a tweet Stu Loeser, Mr Bloomberg's long-time spokesman, said the campaign "didn't know" that "a vendor of a vendor of a vendor used prison labour".

Before setting up a mass media empire, Mr Bloomberg made his billions from data terminals widely used in the financial industry. His net worth is $52bn (£40bn), according to Forbes - 17 times higher than Donald Trump.

Since joining the presidential race, he has spent $59m (£45m) on advertising and recent polls show he has support from 5% of Democratic voters. Former Vice President Joe Biden, Senator Bernie Sanders and Senator Elizabeth Warren are currently leading the race for the party's nomination.

While America's constitution outlaws slavery, the Thirteenth Amendment allows prisoners to be forced into labour for their crimes.

Some labour is voluntary, while some is involuntary and non-compliance can be punished.

In the past, many private firms have used so-called "prison in-sourcing" as an alternative to cheap labour in foreign countries.

Inmates have been used for a variety of tasks like packaging Starbucks coffee and manufacturing lingerie. Prisoners were also paid $1 a day to battle wildfires in California last year.

Supporters argue that the practice boosts local economies and helps to lower reconvictions by providing inmates with skills and income.

But critics argue that prison labour is exploitative and prioritises private-sector profits over the rehabilitation of inmates.

In 2018, incarcerated men and women took part in peaceful prison strikes across 17 US states to draw attention to jail conditions.

As part of their demands, organisers called for an end to "prison slavery" by paying inmates proper wages. Suspected strike leaders were later punished with solitary confinement and the removal of their communication privileges.

Who will take on Trump in 2020?

Mr Bloomberg is one of 15 Democrats vying for a chance to take on President Donald Trump.