The Brexit Party chief said Johnson's ruling Conservatives could pay dearly for rejecting his offer for a "non-aggression pact" in the December 12 snap election.
"There will be no Brexit without the Brexit Party," Farage told a London rally of hundreds of Brexit Party candidates standing in the country's third election in four years.
"Here's the problem with the proposition Boris Johnson is putting to the British people - it is not Brexit," he said to a standing ovation.
Farage's promise of a swift and complete break from the 28-nation bloc brought him victory in European Parliament elections that Britain was forced to take part in due to Brexit delays in May.
The Brexit Party's ratings fell sharply after Johnson seized power in July on a "do or die" pledge to deliver Brexit by October 31.
But Johnson was forced to accept another postponement after failing to push through his withdrawal terms through parliament in time.
His hopes of securing the majority needed to see his deal ratified and Britain leave by the latest January 31, 2020, deadline are now being clouded by Farage's year-old group.
The Brexit Party leader's offer to Johnson came after US President Donald Trump - a "no-deal Brexit" backer who supports both men - endorsed a collaboration.
Johnson has until a November 14 deadline to register candidates to still accept the Brexit Party's "clean break" terms, according to Farage.
But the prime minister said Friday he has "ruled out a pact with everybody because I don't think it's acceptable."
Farage called Johnson's decision "truly regretable".
He also rejected accusations from the Conservatives that he was splintering the pro-Brexit vote.
"We won't split the vote because we'll be the only party offering to leave the EU," Farage declared.
Even the most ardent backers of Brexit have attacked Farage for his decision to contest the vote.
Conservative pro-Brexit lobby leader Steve Baker said Johnson "will not succeed if Nigel Farage creates a hung parliament by dogmatically pursuing purity".
"Whatever Nigel's motives, he risks being the man who threw away Brexit," Baker told the Daily Telegraph newspaper.