Nigel Farage has called on Boris Johnson to ditch his Brexit deal and “build a Leave alliance”.
At the launch of the Brexit Party’s election campaign, the leader said bringing the parties together was “the only way” forward.
But he warned Mr Johnson that if he turned down his offer, the party would field candidates in “every single seat” in England, Scotland and Wales.
The Conservatives have consistently ruled out a formal pact with the party.
It comes after President Donald Trump said Mr Farage and Boris Johnson should team up as “an unstoppable force”.
Mr Johnson negotiated a deal with the EU and tried to get it through Parliament in three days to pass it before the Brexit deadline of 31 October.
However, while MPs voted it through its first hurdle in the Commons, they voted against the short timetable – leading to a three month delay to the deadline.
Mr Farage has been critical of Mr Johnson’s failure to deliver on his promise that the UK would leave by Halloween.
He used the launch to condemn the PM’s deal, urging him to “drop [it] because it is not Brexit”.
Instead, Mr Farage urged him to pursue a free trade agreement with the EU – similar to the deal the bloc has with Canada – and to impose a new deadline of 1 July 2020 to get it signed off.
If an agreement was not done by then, the UK should leave the EU without a deal and move to World Trade Organisation trading rules.
“I would view that as totally reasonable,” he said. “That really would be Brexit.”
But Mr Farage said if Mr Johnson did not pursue the route, the Brexit Party would contest every seat in the country – with 500 candidates ready to sign the forms to stand on Monday.
“The Brexit Party would be the only party standing at these elections that actually represents Brexit,” he said.
The party leader also attacked Labour for a “complete and utter betrayal on Brexit” – and said his party would target Labour seats in the Midlands and North of England.
He said Labour’s plan to renegotiate a deal then put it to a referendum was offering a choice of “remain or effectively remain”.
Mr Farage said there were five million Labour voters who had supported Leave in the 2016 EU referendum, meaning his party “posed a very major problem” for Jeremy Corbyn.
“So many Labour Leave seats are represented by Remain members of Parliament,” he said. “We view those constituencies around the country among our top targets.”
He ridiculed the reported Conservative plan to target “Workington man” – Leave-supporting traditional Labour voters in northern towns – saying Tories needed get out of London more.