Nigel Farage has said that a political pact between his Brexit Party and the Conservatives would be “unstoppable”.
He urged Prime Minister Boris Johnson to leave the European Union without a deal, saying his party would work with the government if it did so.
The Brexit Party leader said a no-deal Brexit was now “the only acceptable deal”.
His comments came after Labour leader Jeremy Corbyn said a no-deal Brexit would put the UK at the “mercy of Donald Trump”.
Mr Farage was speaking at a rally in Westminster in front of hundreds of prospective Brexit Party candidates in a general election.
He said the party has vetted 635 people for any upcoming general election, just 15 short of ensuring it can fight every seat.
He warned his party would fight in every seat at a general election if the government tried to pass the existing EU withdrawal agreement.
Mr Farage said he believed there was a “better than 50% chance” the country would go to the polls in the autumn.
However, if Mr Johnson "summoned the courage" to pursue a no deal, The Brexit Party would work with him, Mr Farage said.
Mr Johnson has promised Britain will leave the EU by October 31 with or without a deal.
Mr Farage said: “A Johnson government committed to doing the right thing and the Brexit Party working in tandem would be unstoppable.
"I want to make this pledge from the Brexit Party. The withdrawal agreement is not Brexit. It is a betrayal of what 17.4 million people voted for.
"If you insist on the withdrawal agreement, Mr Johnson, we will fight you in every seat up and down the length and breadth of the United Kingdom."
However, he added: "If Boris Johnson is prepared to do the right thing for the independence of this country, then we would put country before party and do the right thing.
"We would be prepared to work with him, perhaps in the form of a non-aggression pact at the general election.
"The Conservative Party has lost so much trust that the only way they could win a general election is with our support."
Mr Farage was critical of the prime minister in his address to party candidates, saying ‘a few weeks into his premiership Boris is already going off the rails".
He said: "I'm pleased Boris Johnson has brought some energy and optimism to that job.
"I'm pleased he has said we will leave 'do or die' on October 31. The trouble is we have heard it all before."
He added: "In his letter to Donald Tusk, he said a deal is his biggest priority. He said getting rid of the backstop would make the Withdrawal Agreement satisfactory.
"He voted for the Withdrawal Agreement the third time it came back to Parliament.
"Can you trust Boris Johnson, can you trust the Conservative Party? With or without the backstop, this is still the worst deal in history."
One of the candidates selected for the Brexit Party is a former singer with Eurovision winners Bucks Fizz.
Jay Aston Colquhounis will run to be an MP in the London seat of Kensington, currently held by Labour.
Bucks Fizz won the Eurovision Song Contest 1981 with their song Making Your Mind Up and Ms Aston Colquhounis was in the band from 1981 to 1985.
She said: "I used to live in Kensington and it still feels like home. I want to stand for the Brexit Party and fight to uphold democracy for the sake of the people.
"As an MP, I will work to rebuild trust and help the Brexit Party make the necessary reforms to unite the country after we break free of the European Union."