Vote Leave campaigner Andrea Leadsom last night failed to give an answer on whether a £600,000 donation from a former British National Party member would be returned.
It came to light earlier this week that Vote Leave had accepted the donation from a former member of the far-right BNP.
When questioned on stage last night by TUC head Frances O'Grady about whether the money would be returned, Leadsom said: "That is unworthy of this debate."
"There are millions of people who have very real and genuine concerns about the impact of free movement and who have donated to a campaign to Vote Leave and take back control."
Afterwards she swiftly turned the discussion onto security and immigration.
"Vote Leave took a £600,000 donation from a former BNP member. Are you going to pay it back?" Frances O'Grady https://t.co/jV8koCSkMW— BBC News (UK) (@BBCNews) June 21, 2016
Buzzfeed reported that Vote Leave hinted it was preparing to stand by their donor and keep the money.
Electoral Commission figures released yesterday show Sutton Coldfield resident Gladys Bramall as Vote Leave's third largest individual donor.
The 88-year-old made two separate donations of £500,000 and £100,000.
However, Buzzfeed News reported that Bramall was listed on a leaked BNP membership list.
Bramall told Buzzfeed that she had been unknowingly signed up to party membership by her husband.
A Vote Leave spokesperson said: "This is an 88 year old lady who says she says that her late husband signed her up to the BNP without her knowledge. When she realised what they stood for she says she resigned from the group."
Pressed on the revelations, Vote Leave's Michael Gove responded on LBC that money from "tainted sources" would play no part in the campaign.
Remain backer Sam Gyimah MP described the findings as "extremely concerning".