Paul Nuttall has said Brexit is a “job half done” and UKIP MPs are needed to “see this through to the end” as he launches his party’s election campaign.
Mr Nuttall accused the prime minister of “backsliding” on immigration and said a “whopping Conservative majority” would “put Brexit in peril”.
UKIP would fight seats across “the vast majority of the country” but would stand not against some pro-Brexit MPs.
The party won 3.9m votes in 2015 which resulted in only one seat.
Mr Nuttall said ditching the “antiquated” and “unfair” first-past-the-post voting system would be among “radical” domestic policies in a “fully-costed” manifesto which would show UKIP was not just about Brexit.
But he added: “In many ways it is an election which is on our turf. It is a Brexit election”.
He accused Prime Minister Theresa May of “flagrant opportunism” in calling the snap election for 8 June because the Conservatives were so far ahead of Labour in the polls.
“A whopping Conservative majority will only serve to put Brexit in peril,” he said at his campaign launch in London.
“Hordes of Tory lobby fodder will allow the prime minister to backslide, safe in the knowledge she has the votes banked.”
He accused the prime minister, who campaigned for a Remain vote, of “backsliding” on immigration, not ruling out a “divorce bill” from the EU and not protecting fishermen’s rights, questioning whether she will “get the deal that the British people want”.
“UKIP goes into this snap election determined to hold the government’s feet to the fire on Brexit. We will act as the government’s backbone in these negotiations.”
He added: “I believe we have to see this through to the end. We are only halfway through the war. It is a job half done. We must continue to fight.”
Earlier Mr Nuttall told BBC Radio 4’s Today programme he would do his “duty to lead the party into battle” and stand for election to the Commons although he has yet to confirm which constituency he will contest.
Asked about speculation that he may stand in Boston and Skegness Mr Nuttall told the BBC: “There’s been strong rumours about everything – I’ve also heard Hartlepool, I’ve heard Southport, I think branches across the country”.
Boston in Lincolnshire had the highest majority of Brexit voters in the country in last year’s referendum – more than 75% of voters wanted to leave the EU.
Mr Nuttall said he had asked “a number” of branches to consider not standing in areas currently represented by prominent Leave campaigners from all parties, if they were at risk from a challenge by a pro-Remain candidate.
“I’m not talking about five-to-midnight Brexiteers, or people who have just come to this opinion recently, I’m talking about people who have campaigned with us for many years,” he said.
But he said UKIP would “be standing in the vast majority of the country” adding: “We’ve got 350 candidates already selected, there will be a lot more selected over the weekend.”
He said his predecessor Nigel Farage would “play a front-of-house role in this election” and said he believed the party was “in with a chance of winning a number of seats at this election”.
“We will be targeting sensibly in terms of financial resources and manpower.”
Other policies mentioned during his launch speech were replacing the House of Lords with an elected chamber, cutting the foreign aid budget to fund the NHS, campaigning for an English parliament and tackling the “repulsive” practice of female genital mutilation.
On the accusation from former party donor Arron Banks that UKIP’s proposed burka ban was effectively a “war on Muslim religion”, Mr Nuttall told the BBC the policy would merely bring the UK into line with countries like France and Belgium.
“We are not at war with Muslims. What we want to see is equality, we want to see integration.”
On Thursday, Prime Minister Theresa May warned that “tough” Brexit negotiations ahead meant the Conservatives needed the strongest possible mandate to “strengthen my hand when I negotiate with the prime ministers, presidents and chancellors of Europe in the months ahead”.
Labour set out its approach to Brexit on Tuesday, saying it would scrap Mrs May’s plans and unilaterally guarantee the rights of EU residents before talks start.
For the Liberal Democrats, Tom Brake said: “Paul Nuttall is campaigning on a near-identical platform to Theresa May. It’s surprising he’s chosen to stand instead of going to help draft the Conservative manifesto.”
He said the party was now part of a “regressive alliance” supporting “a hard Brexit”.