"Given where we are, no deal is the best deal," Farage told more than 500 prospective parliamentary candidates from his party, which topped European elections in May.
"If, Mr Johnson, you insist on the Withdrawal Agreement, we will fight you for every single seat" in the next general election, he said, referring to the EU divorce deal currently on offer.
Farage, a leading force behind Britain's Brexit vote in 2016, warned Johnson against attempting to negotiate an amended departure agreement with Brussels, urging him to pursue a "clean break Brexit" instead.
Farage said the Brexit Party was prepared to strike a "non-aggression pact" with Johnson's governing Conservatives in any general election if the prime minister went for the "no-deal" option.
The Conservatives would face electoral annihilation otherwise, Farage warned.
"The only way they could win a general election is with our support," he said.
"We could be their worst enemy or their best friend."
British politicians are still deeply divided over how or even whether to leave the European Union, and the impasse has led to growing speculation that a general election may be imminent.
Johnson's government has a one-seat majority in parliament.
Johnson spoke to European Commission president Jean-Claude Juncker by phone later on Tuesday, while the government's chief Brexit adviser David Frost heads to Brussels for talks on Wednesday.
However, both sides appear reluctant to give any ground.
A Downing Street spokeswoman said the call was "positive and substantive" but that Johnson had reiterated there is "no prospect" of a deal without changes to the withdrawal agreement struck by his predecessor Theresa May.
Meanwhile, the commission said Juncker had warned that any tweaks to the plan must be "compatible" with the deal.
"A 'no-deal' scenario will only ever be the UK's decision, not the EU's," the statement added.
- 'Urgency to act' -
With the clock ticking, Labour leader Jeremy Corbyn met five pro-EU opposition leaders to thrash out a plan to prevent a no-deal Brexit after leaked official planning documents said this could lead to food, fuel and medicine shortages.
"The attendees agreed on the urgency to act together to find practical ways to prevent no-deal, including the possibility of passing legislation and a vote of no confidence," they said in a joint statement.
The pound rose sharply against the dollar and the euro -- by about 0.5 percent -- following the announcement.
Corbyn wanted to call a no-confidence vote in Johnson's government next week and then, if successful, lead a caretaker administration that would postpone Brexit and call a general election.
But the opposition leaders' meeting agreed instead to first try passing a law that would force the government to ask the EU to delay Brexit.
- Suspending parliament an 'outrage' -
Britain's parliament is due to return on September 3.
But anti-Brexit politicians have been discussing plans ever since Johnson came to power last month vowing to take Britain out of the EU on October 31 with or without a divorce deal.
Johnson has said he is hoping for a deal with EU leaders, describing the chances as "marginally" higher following G7 talks over the weekend.
But he has not ruled out suspending parliament -- known as proroguing -- in order to allow a no-deal Brexit if he fails to come to an agreement with the EU in the next weeks.
Some 160 of Britain's 650 MPs have signed what they called the "Church House Declaration" against proroguing parliament.
"Shutting down parliament would be an undemocratic outrage at such a crucial moment for our country, and a historic constitutional crisis," it read.
Britain voted to leave the EU in a 2016 referendum but has already been forced to delay its exit twice after parliament opposed a deal that the preceding PM May struck with Brussels.
Johnson wants Brussels to remove the withdrawal agreement's fallback provisions for the border between EU member Ireland and the British province of Northern Ireland. The Brexit Party wants to junk the entire agreement.