The founder of Leave.EU sent the message in response to a tweet from Green Party MP Caroline Lucas about the 16-year-old’s voyage across the Atlantic on a racing yacht.
Mr Banks shared the post, adding the comment: “Freak yachting accidents do happen in August...”
He has since been blasted by people outraged by his apparent threat aimed at the teen.
Freak yachting accidents do happen in August ... https://t.co/6CPePHYLtu— Arron Banks (@Arron_banks) August 14, 2019
Actress Amanda Abbington tweeted: “If you are a grown up, fully-fledged adult and you are mocking this young girl for trying to save the planet then I genuinely feel sorry for you.
“I also think you are incredibly cruel, vicious and arrogant.”
Caroline Lucas said she had made a formal complaint to Twitter, saying the “vile tweet” and made her “sick to the stomach”.
If you are a grown up, fully-fledged adult and you are mocking this young girl for trying to save the planet then I genuinely feel sorry for you. I also think you are incredibly cruel, vicious and ignorant. pic.twitter.com/zhCjt5ZFd7— Amanda Abbington (@CHIMPSINSOCKS) August 14, 2019
Another Twitter user said: “You know she’s 16, right?” while another described his comment as “despicable”.
TV and comedy writer James Felton described Mr Banks as “some sort of massive d**khead”, saying: “If your reaction to a teenager not wanting the planet to die in a century is to joke about how you wish she’d die now you might just be some sort of massive d**khead just FYI Arron”.
Arron Banks’ vile tweet about @GretaThunberg makes me sick to the stomach— Caroline Lucas (@CarolineLucas) August 15, 2019
I have made a formal complaint to Twitter.
Mr Banks later defended his post and, in response to another tweet which highlighted the fact the Swedish climate change activist was a child, he said: “Quite and one being used.
“Obviously I don’t hope she encounters a freak yachting accident! I just enjoy watching the ludicrous tweeter mob following the next outrage …”
He also replied to someone accusing him of showing “his true colours”, saying: “It was a joke .. you lefties have no sense of humour …”
grown man issuing veiled threats to a kid, this must be a very proud day for you— shauna (@goldengateblond) August 14, 2019
Greta, whose “school strikes” have sparked a global protest movement by young people, is embarking on a two-week voyage across the Atlantic on a carbon-neutral racing yacht, the Malizia II.
The move is so she can attend high-profile climate events without using planes or cruise ships which emit greenhouse gas emissions.