Brexit Party leader Nigel Farage has accused Gordon Brown of “a disgusting smear” in a row over party funding.
At a speech in Glasgow, the ex-PM said The Brexit Party “is making questionable claims about the true source of its funding” ahead of the European elections.
The ex-prime minister has asked the Electoral Commission to investigate.
The Electoral Commission has said it works with all parties to ensure they comply with the law.
Mr Brown highlighted that the commission “has warned of the dangers of multiple, small, anonymous donations being a cover for dirty money”.
Under the rules governing donations, amounts below £500 do not have to be declared.
An official donation of £500 or more must be given by a “permissible donor”, who should either be somebody listed