Ukip intend to be the "guard dogs of Brexit" and ensure Theresa May does not "backslide" on Brexit.
Party leader Paul Nuttall revealed he feared the Prime Minister would not negotiate the post-EU deal the British people "voted for" last June - and predicted she would make concessions on freedom of movement.
Speaking on the Andrew Marr Show, Mr Nuttall insisted that - despite disastrous local election results earlier this month - Ukip remains "strong" in its traditional areas of support.
And he predicted a bright future for the party, claiming it could grow bigger than it ever has before if the government fails to deliver on Brexit.
Mr Nuttall told Marr that "clear water" divided Ukip from other parties - especially on issues such as foreign aid, which if elected Ukip would scrap and redirect its funds towards the NHS.
He said: "We're the only party truly committed to cutting immigration and of course we have to be the guard dogs of Brexit.
"Because I fear, once this election is over and Theresa May comes back with a whopping majority, that she will begin to backslide.
"And that's why it's important Ukip is there to ensure we get the Brexit we voted for on June 23."
He continued: "Because if she [May] begins the backslide - and I think she will barter away our fisheries, I think there will be a deal on freedom of movement, I think there will be a divorce bill - people did not vote for that last year."
Asked whether he believed it was time Ukip "wound up" following a disastrous set of local elections on May 4, Mr Nuttall insisted it was not.
"Ukip has a great future and I predict that after this election Ukip could be bigger than it has ever been before," he said.
Mr Nuttall added: "What Ukip needs to do is to be electorally viable, it needs to be going up the polls, it needs to be doing well at local elections night."
Describing his experience of canvassing for the June 8 election, he said: "Where we're strong, we're still strong."