Ukip must "own the immigration argument", Nigel Farage has told ITV News, as he said failed tactics had cost Paul Nuttall victory in the Stoke by-election.
The former party leader said he was "disappointed" at his successor's defeat as Labour held the seat and said a failure to be "tough enough" on immigration cost them votes.
Mr Farage said Ukip must remain a radical party and be more "out there" as he warned against a move towards mainstream political debate.
"Ukip is a radical party or it is nothing," he said. "There are voices in Ukip that want us to look a bit more like the others."
Mr Farage said his party should capitalise on the start of Brexit process.
"Net migration is still running at huge levels," he said.
"I think there is a very great danger that once (the) Article 50 (EU exit mechanism) is triggered it may lead to a great rush of people.
"I think Ukip needs to own the immigration argument."
He said his party had lessons to learn on its "messaging" and "tactical campaign", but added: "I'm certain Paul Nuttall will learn them."
Mr Farage had earlier called for a tougher stance on immigration, saying: "We will have to look at that and think: were we really tough enough, were we clear enough with the electorate? It has got to be looked at."
He said he felt "sorry" for Mr Nuttall after the Ukip leader's failure to win the Stoke-on-Trent Central battle triggered by the resignation of former shadow education secretary Tristram Hunt.
Ukip had seen Stoke - which voted overwhelmingly for Brexit in last year's referendum - as fertile ground for a challenge to Labour.
But Mr Nuttall's gamble of standing himself failed to come off as Labour's Gareth Snell held the seat with a majority of 2,620.
It has raised questions as to whether Ukip could take seats from Labour in its traditional heartlands.
Party chairman Paul Oakden said it could be another two decades before the party can take another seat in a by-election.
Stoke had been regarded as promising territory by Ukip.
Mr Oakden acknowledged Mr Nuttall had endured a "difficult" campaign, in which he had to apologise over a false claim that he lost close friends in the Hillsborough disaster, but said he had the full support of the party.
He said: "This party is absolutely behind Paul Nuttall as its leader. He is 12 weeks into his leadership.
"We are all going to support him moving forward. This is one step along a long road for our party," he said.
Mr Farage also defended Mr Nuttall over the Hillsborough row, saying: "He had a tough time. He paid the price for some mistakes that staff of his had made. I don't think that this reflects on Paul."