Police are investigating a crowdfunding page which sought to raise £10,000 to have campaigner Gina Miller killed.
The GoFundMe page targeting Ms Miller, who twice won Supreme Court cases challenging the government on Brexit, was first posted in April.
Police said the fundraising campaign was reported to them on Wednesday.
GoFundMe took down the page, which had not raised any money, and apologised for any distress to Ms Miller, who called the crowdfunding "horrifying".
Speaking to the Sunday Mirror, which first reported the police investigation, Ms Miller said: "This is horrifying. It beggars belief that this can have been allowed to have been put up on this site and stayed there for so long."
A Met Police spokesman said: "Officers from the Met's south west CID team are currently investigating a report of threats to kill that was reported to them on Wednesday October 23.
"Enquiries remain ongoing and the victim, a female aged in her 50s, has been regularly updated."
A GoFundMe spokesman said the company was sorry this campaign "got through our otherwise robust procedures".
"We are particularly sorry for any distress this caused Gina Miller," he said.
Ms Miller, a 54-year-old investment manager and philanthropist, stepped into the spotlight in 2016 when she launched a crowdfunded legal challenge to the government.
It forced the government to give MPs a vote on invoking Article 50 and triggering the Brexit process.
In September, she won a landmark Supreme Court case establishing that Boris Johnson's suspension of Parliament was unlawful, because it prevented MPs carrying out their duties in the run up to the Brexit deadline.
But since her first court victory, she has suffered online abuse, including rape and death threats against her and her family, prompting her to employ round-the-clock security.
Responding on Twitter to the crowdfunding threat, Ms Miller said: "We need to heal our nation and my view is that the only way of doing that is to remember true British values of tolerance, decency, reason, civic duty, common-sense and, above all else, honesty and kindness."
Labour MP David Lammy tweeted that the threats were "despicable", adding that Ms Miller had "tirelessly fought for British values". "What is happening to our country?" he asked.