Nigel Farage says he will enter into a general election pact with Boris Johnson but only if the prime minister scraps the existing EU withdrawal agreement.
The Brexit Party leader said he would be willing to enter into a non-aggression pact with the Conservatives if Mr Johnson abandons Theresa May’s deal.
Mr Farage suggested he would enter a pact with the Tories if Mr Johnson either pursues a no-deal Brexit or negotiates a brand new deal with Brussels.
However, if a snap general election is called before October 31, Mr Farage said his party is “ready to go” and will field candidates in every seat available.
Mr Johnson has vowed to scrap the Irish backstop preventing a hard border between Northern Ireland and the Republic of Ireland if the UK leaves the EU.
Mr Farage told The Times: “If Boris Johnson thinks that the withdrawal agreement can stand then we don’t.
“At the moment he is suggesting no change at all to the withdrawal agreement other than the Irish backstop. This is not acceptable.”
There are fears among Conservatives that without a pact, the Brexit Party could split their vote in a general election.
On Monday night, Mr Farage predicted that Mr Johnson will lose Tuesday's vote forcing a Brexit extension and the country will face a general election in October.
Speaking at a rally in Colchester, he said: "They, allied with us, would be unstoppable in the general election. We could get Brexit, we could destroy Corbyn and we would be a free, independent nation.
"Yes, there may be on November 1 one or two bumps in the road, but free and proud we'd be able to get on with the rest of our lives.
"Boris, you can't win without us. Boris, if you go for the withdrawal agreement I'm afraid you will lose, but if you do the right thing and we support you you'll be a national hero in the history of this nation."
Earlier on Monday, Mr Farage told LBC Radio: "The big message here is Boris Johnson wants the Withdrawal Agreement. He wants Mrs May's Withdrawal Agreement, but minus the backstop.
"And if he does get voted down in the House of Commons tomorrow, which I have to say, looking at the Tory rebels who are beginning to stand up, does look quite likely to me, then there will be a general election.
"Do I think it is acceptable that a general election could be held on the basis that he goes back to Brussels to get the Withdrawal Agreement? Well I absolutely don't."
"Given there is less than 60 days to go to that right now, no-deal is the best deal, getting out with a clean break Brexit and after we will get a trade deal.”