Billionaire and US presidential candidate Michael Bloomberg admitted that his campaign headquarters unknowingly used the work of prisoners to make phone calls on his behalf, Reuters reported.

According to Bloomberg, cooperation with the company that used the prisoners was terminated. 

"We do not support this practice and we are making sure our vendors more properly vet their subcontractors moving forward," Bloomberg said in a statement. 

Bloomberg, who ranks eighth in the list of the richest Americans, only took part in the presidential race in November, but he had already spent more on campaign advertising than his main rivals in the struggle for the nomination from the Democratic Party for the whole year.

"A Reuters/Ipsos opinion poll conducted on Dec. 18-19 showed about 5% of Democratic-leaning voters support the billionaire former mayor of New York. Recent polls show former Vice President Joe Biden, Senator Bernie Sanders and Senator Elizabeth Warren are the party’s leading candidates," the source added.