Jeremy Corbyn is "completely bonkers", Wikipedia founder Jimmy Wales has said.
His comments came as the frontrunner for the Labour leadership came under attack from rivals Andy Burnham, Yvette Cooper and Liz Kendall for floating the idea of women-only carriages on trains.
Wales, who lives in London, frequently tweets about politics and the Labour Party leadership contest. On Tuesday he said: "I wonder how many people sent in their ballots for @jeremycorbyn and now deeply regret it."
And added by way of explanation: "Because on a wide range of issues, it's become clearer and clearer he's completely bonkers."
Wales was also critical of the Labour leadership process, which allows members to case their votes throughout August and early September. "Unfortunate to have a long balloting period when news about the candidate's positions is still coming out," he said.
Instead, the Wikipedia founder said voting should be conducted on one single day at then end of the contest, just as with a general election.
On Wednesday it emerged that Mark Serwotka, general secretary of civil servant workers' union PCS and leading Corbyn supporter, has been banned from voting in the contest.
More than 130 staff have been hired by Labour to verify votes from the some 366,000 people who signed up in the aftermath of May's general election, after concerns were raised over 'entryism' from both the left and the right.
But Serwotka, who was a Labour member until the late 1980s, said he had not joined any other party since leaving, and claimed to have been unfairly ousted from the balloted electorate.