Scottish MP Natalie McGarry faces possible legal action after accusing the leader of a pro-union campaign group of holocaust denial.
In a now-deleted series of tweets, McGarry said Scotland in Union was headed by an "internet troll" and "outed holocaust denier". She has since apologised but BuzzFeed News has learned legal action is being considered.
Scotland in Union, which describes itself as a "non-party movement which unites people around a positive view of Scotland in the UK", is led by Edinburgh-based management consultant Alastair Cameron.
McGarry was elected as an SNP MP last May but stepped down from the party due to questions over the funding of the Women for Independence campaign group she helped set up. She is now an independent MP for Glasgow East.
In her tweet on Sunday morning, McGarry wrote: "Leading Unionist figures do know that 'Scotland in Union' is headed by an internet troll and known holocaust denier, right?"
The Scotland in Union Twitter account replied: "Very surprised by this unfounded claim, have you looked at our website for more information? Good thing to do before you tweet."
McGarry wrote back to say she had deleted the tweet, but continued: "I'd add however, that you should be careful who is distributing your materials, but apologies etc."
A spokesperson for Scotland in Union told BuzzFeed News Cameron was consulting a lawyer on Monday morning, saying: "We are surprised that an elected member of parliament would make such a false and offensive accusation. We will be taking legal advice before responding formally to Ms McGarry."
The spokesperson later said: "We can confirm that Alastair Cameron has instructed a legal firm to engage Ms McGarry and seek an appropriate apology and a charitable donation."
McGarry previously apologised to JK Rowling after the author threatened her with legal action over accusations from the MP that Rowling supported misogynistic trolling.
The MP's office did not respond to a request for a comment.