The Liberal Democrats have suspended a member of staff after confirming the person sent "incorrect information".
Party leader Jo Swinson told ITV News their actions had been "absolutely unacceptable".
Earlier this week, it was alleged that one of their staffers had forged a back-dated email to try to back up the party’s threat of legal action against the OpenDemocracy website.
The website claimed the Lib Dems had been selling voter data to the Britain Stronger in Europe group, which later became the People’s Vote campaign.
Ms Swinson said: "There was an email that was sent which was inaccurate, which was faked and that's not acceptable, there is an investigation and the member of staff has been suspended."
Ms Swinson added: "That particular incident is absolutely unacceptable which is why we have taken this swift action and are investigating.
"I'm not for a second going to say that, that is acceptable and it is right that we have taken that action."
A Liberal Democrat spokesperson told ITV News: "The Liberal Democrat’s refute allegations made in OpenDemocracy's piece of 13 November.
"However, we have been made aware that the information OpenDemocracy subsequently received from the Liberal Democrats was incorrect.
"We have suspended a member of staff involved and are following due process."