Brexit Party leader Nigel Farage has said he is open to an electoral pact with the Conservative Party - if Boris Johnson is genuine about taking the UK out of the EU on 31 October.
Mr Farage said Mr Johnson would need to call an election if he wanted a no-deal Brexit, in order to "change the arithmetic" in the Commons.
He said there was then a "possibility" of a pact between the parties.
But he added: "I don't believe a single word the Conservative Party tell us."
An electoral pact usually involves not fielding candidates in specific areas, in order to allow another party a better chance of winning.
A pact between the Brexit Party and the Conservatives could avoid splitting the pro-Brexit vote - but Mr Johnson has previously said he does not believe the Tories should do deals with any other party.
The Brexit Party was the clear winner in the UK's European elections in May, taking almost 32% of the vote in Great Britain, with the Conservatives winning only 9%.
Mr Farage said: "Theresa May told us 108 times we were leaving on March 29 and we didn't, so just because Boris says we're leaving on the 31 October doesn't mean we're going to."
"We would need to believe them and at the moment that's not very easy," he added.
Mr Johnson - who was elected Tory leader and the UK's next prime minister on Tuesday - has pledged the UK will leave the EU on 31 October "do or die", and with or without a deal.
Asked at a Tory leadership hustings last week whether he could work with the Brexit Party, Mr Johnson said: "I don't believe that we should do deals with any party."