Ian Austin, one of former Prime Minister Gordon Brown's closest allies, told the BBC that the Labour Party has been poisoned by "anti-Jewish racism" under Corbyn's leadership. Austin left the party in February over its handling of an anti-Semitism scandal.
"There's only two people who can be prime minister on December 13 - Jeremy Corbyn or Boris Johnson," Austin told the BBC. "And I think Jeremy Corbyn is unfit to lead our country."
The startling interview came a day after deputy Labour leader Tom Watson, who has often clashed with Corbyn, announced he was stepping down. The two actions underscore the unease of many of Labour lawmakers with Corbyn's left-wing views and his ambivalence over Britain's ties to the European Union.
Watson acknowledged his "political differences" with Corbyn in a resignation letter, but said the decision was personal rather than political. But his departure is likely to embolden others who have been uneasy with Corbyn's leadership since he took charge of the party in 2015, moving it further to the left and away from the center ground staked out by former Prime Minister Tony Blair.
Before he won the party leadership, Corbyn was a long-time backbench lawmaker who championed leftist causes and the grievances of groups such as the Irish Republican Army, Hamas and Hezbollah.
"I think he's spent his entire time in politics working with and defending all sorts of people, extremists and in some cases anti-Semites and terrorists," Austin said. "In the end, I don't think he's a patriot. I don't think he loves his country. I think he always picks our country's enemies, whether it's the IRA during the Troubles or describing Hamas and Hezbollah as his friends, or parroting (Russian President Vladimir) Putin's propaganda when the Russians send hitmen to murder people on the streets of Britain."
Labour was quick to hit back. Rebecca Long-Bailey, the party's business spokeswoman, told the BBC that Corbyn is a patriot and the party has stepped up efforts to root out anti-Semitism.
"Certainly voting for Boris Johnson if you are a Labour voter and you want to protect your community is absolutely absurd and it makes no sense at all," she said.
Britain's biggest Jewish newspaper, the Jewish Chronicle, will on Friday use its front page to appeal to non-Jewish voters not to cast a ballot that could put Corbyn in Downing Street.
Here's our front page this week - addressed not to our usual readers but to our fellow British citizens pic.twitter.com/7K5qi8GhBJ— The Jewish Chronicle (@JewishChron) November 7, 2019
"Throughout his career he has allied with and supported antisemites," the paper says.
"If this man is chosen as our next prime minister, the message will be stark: that our dismay that he could ever be elected to a prominent role in British politics, and our fears of where that will lead, are irrelevant."