Nigel Farage has said the UK is a "complainants' society", as he faces ongoing questions about both his and his party's finances.
Speaking to the BBC ahead of the party's annual conference this week, the Reform UK leader said a "whole ecosystem" didn't want "new kids on the block succeeding".
It comes after a summer dominated by headlines about the party's finances and £5m he received from a party supporter before the last election.
Farage acknowledged his party's poll ratings had taken a hit compared to last year, but said it would rebound and still had "every chance" of winning the next general election.
Reform's annual conference begins in Birmingham on Thursday, with both Farage and the party facing renewed questions over money.
A parliamentary investigation into £5m he received from major Reform donor Christopher Harborne was relaunched following Farage's re-election in Clacton, after a by-election he called in the Essex seat.
The Metropolitan Police is also investigating whether payments to Reform in the weeks before the last election complied with donation laws.
The police probe is examining donations given directly to the party by Fiona Cottrell, mother of longstanding Farage ally George Cottrell.
It is also examining payments from a company controlled by Reform's deputy leader Richard Tice, which received £1m from Fiona Cottrell in June 2024.
Asked if he knew whether or not money donated by Fiona Cottrell had come from her son George, Farage said he was not "involved in the transactions".
However, he added that he "had been told it's her money". Reform has insisted it has always complied with donations rules.
He added: "The point is, we live in a complainants' society. If you complain about me, automatically that triggers an investigation."
He said that political parties were "constantly writing to all the authorities", meaning Reform was "under investigation every day for virtually everything we do".
"There's a big picture here. The old two-party system that has dominated since the end of the First World War feels for the first time it faces an existential threat.
"There is a whole ecosystem with a century of development around this. And we come along from virtually nowhere and threaten the whole thing.
"So I think what you're seeing is the establishment, in a very co-ordinated way, fighting back."
Pressed on what he meant, he added: "I'm talking about normal human behaviour.
"People who hold positions of power, influence and money don't want new kids on the block succeeding. It's the whole ecosystem, it's everything."
Farage said he expected it "won't be too long" before he is interviewed by Parliament's standards commissioner Daniel Greenberg as part of his probe into whether he should have publicly declared the £5m he received from Harborne.
He continued to insist the sum, which he says was a personal gift, did not require registration under Commons rules.
He added he had "no questions" about Greenberg's impartiality, but hit out at the "political composition" of the standards committee that will recommend what action should be taken on his findings.
The 14-member panel includes seven MPs from Labour, the Conservatives and Liberal Democrats, but none from Reform UK.
He added: "Have you seen who's on the committee? I think we know the outcome already. I'm not going to say anything more than that."
"To be judged by members of Parliament by other political parties who you're fighting against in by-elections or general elections, I think that raises a question, I really do," he added.
Elsewhere in his interview, Farage acknowledged that Reform was "down a bit" in its poll ratings compared to 2025, after failing to win in by-elections this year including Gorton and Denton and Makerfield, which saw Andy Burnham's re-election as an MP ahead of becoming Labour leader.
But he added that "markets don't move in straight lines", adding that the party still had "every chance" of winning the next general election, due by 2029.
He said the party was continuing to look "outside of the political sphere" to fill frontbench positions, following a raft of appointments earlier this year to senior posts within the party.
And he said he continued to hold to his previous prediction there would be an election next spring "forced on the government" by the markets, amid rising costs to public borrowing.
Reform's conference, he said, would be a chance for the party to set out details of the "fundamental changes" it would make within 100 days of winning power.
He added that whilst his and other parties had announced a number of new policies in recent weeks, what most interested voters was who is "actually going to deliver" than the specifics of individual policies.
