He was elected by a wide margin to be the Labour Party's leader last year, after Mr Ed Milliband resigned due to the party's crushing defeat at the general election.
Now Mr Jeremy Corbyn is facing a coup within Labour barely a year later.
Shadow ministers who quit on him - 20 at last count - all said he was a principled, decent person but not a leader who they believed could lead them through this Brexit crisis and a possible early general election.
Mr Corbyn, 67, commands much support from the trade unions, where the party has its roots. He started out as a representative of various trade unions before being elected to the Haringey Council in 1974 in North London.
He won a seat in the 1983 general election under the Labour ticket in his local Islington North constituency - a seat he has held on to for the past seven elections.
Known for being a human rights, anti-war, anti-nuclear activist, he often refused to toe the party line when it was under the "New Labour" governments of Mr Tony Blair and Mr Gordon Brown.
He famously said in 1984, at the suggestion that "Labour scruffs" who wear open-necked shirts and jumpers, which he favours, should be banned from the House of Commons: "It's not a fashion parade, it's not a gentleman's club... it's a place where the people are represented."
He is known for having one of the lowest expenses among MPs. He rides a bicycle and is a vegetarian. He also believes the monarchy should be abolished.
Party members are divided over Mr Corbyn's brand of left-wing politics. Some feel he has brought the party closer to its original socialist principles, but others accuse him of being an outdated politician.
He barely scraped through with the minimum number of nominations from MPs needed - 35 - to enter the leadership race last year, but went on to win by a large margin of nearly 60 per cent from the party's 380,000 members.
MPs who nominated him later expressed regret, disappointed at what they said was his weak leadership.
That manifested itself during the EU referendum campaigning, when Mr Corbyn failed to convince staunch Labour supporters in Wales, Yorkshire and north- east England to vote to stay within the European Union.