John McDonnell has compared Reform UK leader Nigel Farage to Adolf Hitler, with critics accusing the former Shadow Chancellor of being "out of touch with reality".
Mr McDonnell called Reform a "fascist organisation" in comparison to the Nazis, describing Mr Farage as a "demagogue".
He said Reform's attitude to asylum seekers is like that of the Nazi's towards the Jews.
Speaking at an event at the Trades Union Congress (TUC) conference, the MP said: "Reform is a protest fascist organisation. We've seen it in the 30s.
"What they do, they have a demagogue speaking for them, they target a particular group, in the thirties in Germany, it would have been the Jews, here it is asylum seekers.
"We've seen it all before. So what do you do? Well, you have to combat it. You have to combat the arguments. You have to mobilise against them."
Deputy leader of Reform, Richard Tice, described the remarks as "both disgusting and indefensible, but sadly typical of someone so out of touch with reality".
Mr McDonnell lost the Labour Whip in July last year after he defied the Government by voting to scrap the two-child benefit cap.
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John McDonnell compared Nigel Farage to Adolf Hitler
| PACommenting on the rise of Reform, the former Shadow Chancellor said: "What's Labour doing? Labour's dancing to Farage's tune.
"I hide under the duvet sometimes in the morning, I don't want to turn on the news to hear another Labour announcement that we're attacking asylum seekers in some way."
Mr McDonnell later denied the comparison of Mr Farage to Hitler or Reform to the Nazis.
He said he was "warning about the situation as it is and as it could be".
Mr Tice stated: "Our country is being overrun by an influx of unvetted, unchecked, fighting-age men. They are costing the taxpayer extraordinary sums and placing an enormous strain on public services.
"The overwhelming majority of the British public agrees. By pledging to end the invasion of our borders and deport those who have no right to be here, we are simply upholding the wishes of the people.
"This is not an attack on asylum seekers. It is about putting the British people first."
In May, the Prime Minister's Attorney General, Lord Hermer, compared threats to "abandon" international law, like the European Convention on Human Rights (ECHR), to Nazi Germany.
Richard Tice described the comments as 'disgusting and indefensible'
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He backtracked on his comments after receiving major backlash, calling his remarks "clumsy".
Mr McDonnell told the TUC meeting: "That's the political argument. Actually, we're going to lose a Labour Government if we're not careful, but we're going to lose it [to Reform].
"That means a dangerous moment for many of our communities, a dangerous moment for all of us."
The conference in Brighton has been debating how unions can deal with the rise of Reform UK.
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