Arron Banks has been criticised after he appeared to wish harm upon Greta Thunberg as the 16-year-old activist set sail across the Atlantic on a two-week voyage in a zero-carbon yacht.

The controversial Brexit backer warned the teenager that “freak yachting accidents do happen in August” as he responded to a tweet by Green party MP Caroline Lucas who said Thunberg was carrying “the vital message to the UN that time is running out to address the climate emergency”.

His comments sparked outrage among MPs, celebrities and academics, but he defended the remark in a series of late-night tweets on Wednesday in which he accused “lefties” of having no sense of humour.

Arron Banks (@Arron_banks) Freak yachting accidents do happen in August ... https://t.co/6CPePHYLtu

Labour MP Tonia Antoniazzi was among the first to rebuke the former Ukip donor and pay tribute to the work Thunberg has done to raise awareness around the impending ecological crisis.

“I think it is absolutely disgraceful that a man like Banks can take to Twitter to slag off a 16-year-old girl,” she told the Guardian. “The great work that Greta has done to put climate change at the top of the political world’s agenda, mobilising and inspiring young people and our future generations is priceless.

“This is a man who funded Brexit lies, lives a life of excess and chooses to wish an accident on Greta Thunberg. It’s an absolute disgrace, his tweet should be reported as a hate crime.” She said Banks should have “opened his wallet and offered to pay for the journey for Greta. The sewer he chooses to swim in is not one which we should have to expose our children to.”

Prof Tanja Bueltmann, founder of campaign group EU Citizens’ Champion, said Banks had “invoked the drowning of a child” for his own amusement.

“Verbal attacks on Greta Thunberg have been coming from many different people for quite some time. Their common characteristic? Most of them are white middle-aged men from the right of the political spectrum.

“As if the tweet itself wasn’t horrendous enough already, Banks’s explanation for it is that he invoked the drowning of a child for fun because he enjoys watching outrage. I cannot think of anything lower than that.”

Play Video 1:40 Greta Thunberg begins zero-carbon Atlantic voyage – video

Actor Amanda Abbington tweeted: “You’re wishing a potentially fatal accident onto a sixteen year old girl, why..?”

Earlier, she wrote: “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.”

In response to a tweet which drew his attention to the fact she is a child, Banks replied: “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.”

Later, replying to a tweet which said he had “finally showed his true colours”, he said: “It was a joke .. you lefties have no sense of humour.”

• This article was amended on 13 September 2019 to correct wording of a link to a related story stating that the yacht was solar-powered. The yacht was mainly powered by wind.