Liberal Democrat leader Jo Swinson has confirmed a member of her party's staff has been suspended for "faking" an email.
The email was reportedly sent as part of a legal fight between the Lib Dems and news site Open Democracy.
Ms Swinson said the "unacceptable" incident had led to "swift action".
The row between the Lib Dems and Open Democracy relates to a story about the party selling personal data - something the party denies.
The Lib Dems accused the website of not including a response from the party in its story.
The website insists one of its journalists had contacted the Lib Dems for a response ahead of publication, but the party had not replied.
Privacy watchdog report
According to Open Democracy's editor, the party then produced a copy of an email, through its lawyers, to back up its claim that it had sent a response.
But the email was dated 18 hours before Open Democracy had asked for a response, suggesting it was a fake.
Ms Swinson did not confirm the identity of the person involved.
But she said: "As has been reported, there was an email that was sent which was inaccurate, which was faked.
"That's not acceptable, there is an investigation, the member of staff has been suspended and I'm not going to comment further on staffing matters."
Open Democracy reported that it had seen evidence held by the UK's privacy watchdog about the alleged sale of data by the party to Britain Stronger in Europe, the official Remain campaign in the EU referendum.
In a 2018 report, the Information Commissioner's Office (ICO) said it had "obtained information" to show that the personal data was sold by the Lib Dems to the Remain campaign for £100,000.
The ICO said the Lib Dems and the campaign group denied that party members' data had been sold.
Both insisted that Britain Stronger in Europe had bought information on the official electoral register from the Lib Dems, the ICO said.
A Lib Dem spokesman said: "The Liberal Democrats refute allegations made in Open Democracy's piece of 13 November.
"However, we have been made aware that the information Open Democracy subsequently received from the Liberal Democrats was incorrect.
"We have suspended a member of staff involved and are following due process."