The International Trade Secretary said the Corbyn-supporting Momentum movement were “extremists” and warned of the “damage” it could do to the country.
Dr Fox also used an appearance at a Conservative Party conference fringe event to warn about the dangers of protectionism in an apparent reference to Donald Trump.
In one of the strongest attacks yet by a Tory minister on Mr Corbyn, Dr Fox said it would be a mistake to dismiss Labour as an unelectable joke.
“I do not find Jeremy Corbyn funny. I think it’s extremely dangerous for the Conservative Party or any other political grouping to say that this party has now taken such leave of its senses that it couldn’t possibly be elected,” he said.
“Electoral circumstances are unpredictable, we know that from history. This is a very dangerous leader of a very dangerous party at the present time, a party that doesn’t believe in Nato, that would abandon our nuclear weaponry, that doesn’t believe in most of the accepted rules of our economics.
“This is a very dangerous party indeed and we should not at any point stand back from pointing out how dangerous they actually are.”