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 imposing a new deadline of 1 July 2020 to get it signed off.
"I would view that as totally reasonable," he said. "That really would be Brexit."