FILE PHOTO: Democratic U.S. presidential candidate Michael Bloomberg delivers remarks where he was honored by the Iron Hills Civic Association at the Richmond County Country Club in Staten Island, New York, U.S., December 4, 2019. REUTERS/Andrew Kelly

WASHINGTON (Reuters) - Billionaire U.S. presidential candidate Michael Bloomberg on Tuesday said his campaign had unknowingly used prison workers to make telephone calls on his behalf.

Bloomberg, who last month entered the Democratic Party race to face Republican President Donald Trump in the November 2020 election, said the campaign had ended its relationship with a company that used prison labor for making phone calls.

“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 said the campaign learned of the ties to prison labor when a reporter called. The Intercept, a news website, was first to report the use of prison labor.

Ranked by Forbes as the eighth-richest American, Bloomberg has spent more on campaign ads in the last few weeks than his main Democratic rivals have all year.

He has so far failed to crack into the top tier of candidates in public opinion polls.

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.