In a note on the site in December, the boss, Hamish McKenzie, said the firm "doesn't like Nazis" and wished "no one held these views". But he said the company did not think that cutting off cash for sites that publish extreme views was the answer, and it would only make things worse.

However, some of the biggest newsletters on the service have said they will leave if Substack does not change its mind.

Casey Newton, who writes Platformer - a popular tech newsletter on the platform with thousands of subscribers paying at least £7 a month - became the latest to quit.

Substack gets a 10 per cent slice of subscriptions from paid newsletters, so losing Platformer could cost them a fortune. Other newsletters have already left. Talia Lavin, a journalist with thousands of paid subscribers on her newsletter The Sword and the Sandwich, moved to a rival service, Buttondown, on Tuesday.

Substack's top team said in a statement: "As we face more pressure to censor content on Substack that some find dodgy or offensive, our answer is the same: we make decisions based on principles not PR, we will stand up for free speech, and we will keep our hands off content moderation."