The Brexit Party leader said he could throw his support behind Mr Johnson if he had the “guts” to call an election aimed at taking the UK out of the European Union without a deal.
Mr Farage – whose newly-formed party came first in last month’s European elections – has claimed in recent days that he has been approached by Conservative Party donors interested in forming an electoral pact.
But the former Ukip leader went further at a Telegraph Live event on Tuesday night, saying he would “of course” work with a new Conservative leader committed to a no-deal Brexit.
Asked whether he could form a pact with the Tories if Mr Johnson wins the party’s leadership race, Mr Farage said: “If he has the guts to be prepared to be voted down by Parliament, to call an election on a clean Brexit, then I would suggest that… logically there is a deal that could be done, and in those circumstances Boris would come back with a massive, massive majority.”
The Brexit Party leader added: “If he is prepared to be voted down and go to a general election on that ticket then with the support of people like me he would win a massive, thumping majority…
“If he was prepared to do that of course I’d want to work with them, of course I’d do that.”
The Brexit Party – which is committed to taking the UK out of the European Union without a deal – gained 29 MEPs at last month’s EU-wide elections, hoovering up 31% of the vote.
The Conservatives meanwhile slumped to fifth place, losing 15 MEPs and taking just 9% of votes cast.
Mr Johnson told Tory MPs earlier this month that the party faced “extinction” unless it delivered Brexit and put Mr Farage “back in his box”.
And he said: “There is a very real choice between getting Brexit done and the potential extinction of this great party – but I believe I can take on Farage and win back the voters being won over by him.”