Nigel Farage accused Keir Starmer of dragging the UK back into the European Union by stealth and predicted voters will punish Labour for it.
The Reform UK leader said "more and more people will come towards us" as the Prime Minister is "creeping back" towards Brussels.
Ministers are considering allowing EU courts a say over meat and dairy imports, prompting fury amongst those who voted to leave the EU.
But Mr Farage - speaking to the Daily Express after parking Reform's "tanks on the lawns of the Red Wall" - warned voters it is an "indication of where we might be going".
And the architect declared Labour will "oversee record numbers of illegals crossing the Channel", as he set out his plans to tackle immigration, leave the European Convention on Human Rights, industrialisation, council spending, tax and benefits in a wide-ranging speech in County Durham.
Mr Farage, speaking in Newton Aycliffe, one of the areas Boris Johnson targeted in 2019 with the Get Brexit Done slogan, declared of Sir Keir: "Clearly, as far as the EU is concerned, he's creeping back in little bit by little bit.
"What we saw yesterday with the ECJ was a sort of indication of where we might be going. "If that trend continues, and people wake up to it, yeah, they'll pay a heavy price. More and more people will come towards us."
The Reform UK leader added that Sir Keir will "oversee a record number of illegals coming to the UK". More than 700 migrants are feared to have crossed the Channel on Tuesday, breaking the record for the number of arrivals on a single day this year.
Slamming Labour's record on immigration, the Reform leader told supporters: "Channel crossings so far this year are over 40% higher than they were last year, and that's before we get the warm weather. That's before temperatures in the Channel increase.
"So we're on for a record year and there's no sign of anyone crossing the Channel being deported. Nobody that comes is going to be deported because, and the Prime Minister made very clear in only his second speech in the Commons as Prime Minister, of how wedded he was to the European Convention on Human Rights.
"As far as we are concerned, this is about sovereignty, and we will leave the ECHR if we're in Government, we'll repeal the Human Rights Act and stop our now somewhat corrupted judiciary from using this at every opportunity to not deport people."
Reform's support since the last election has soared from 18 to 30%. At the same time, Labour's vote share has plummeted from 39 to 27%. Mr Farage believes his party is attracting "life-long Labour voters" and accused Sir Keir's party of abandoning the working class.
He told the Daily Express: "It's not safe going out on a Friday or Saturday night, if you're a girl particularly. The tax burden gets bigger and bigger and bigger. You just feel the country doesn't quite offer the opportunities that it should.
"That can be turned around, an economic turnaround, a societal, attitudinal turnaround, can make it a different place. At the moment, I just think we're in decline.
"People are very, very pessimistic about the future. Going abroad has become a very attractive option, and it is amazing to think, Portugal, Italy, they are looking for young, bright people to go and move and many are."
Mr Farage said of his party targeting swathes of councils in the Midlands and Northern England: "Reform are parking their tanks on the lawns of the red wall. Today's the first day I've said that but I absolutely mean it, and we're here, and we're here to stay.
"And the evidence is that people who are switching to us, this is not a short term protest. They actually believe in us."
He added: "If we go on like this, it'll be 'Nige-mare on Downing Street'."
Mr Farage branded Labour "a very middle class party dominated by human rights lawyers who seem to care more about vague concepts of international law coming from foreign and in some cases, in my view, illegitimate courts, than it does about working people".
But with support surging, questions are already being asked about what a Reform council will look like. And the Reform leader vowed to slash spending on diversity, equity and inclusion (DEI) and "ergonomic chairs".
Mr Farage, asked how he could guarantee that voters would not end up with the same frustrations they had under Labour and the Conservatives, said: "No political party can ever guarantee anything but you can say what you're going to do and then be judged on whether you've done it or not.
"I think point number one is frankly we've got to get the auditors in, we've got to see the long-term contracts that many of these county or unitary authorities have signed up to.
"We will look at the sums spent on DEI, we will look at the half-a-million quid being spent on ergonomic chairs for staff of Lancashire County Council.
"We will look at the £18,000 spent on driving lessons for cross-Channel migrants, and I could go on with a list as long as your arm."
Mr Farage, discussing the plans for an Elon Musk-style Department Of Government Efficiencies , added: "I've been going around the country, at every county I've been to, reading up on lists of money, public money, that has been spent.
"Why are Cornwall spending £10.7 million pounds last year on agency staff, whilst at the same time they have a work from home culture? Hardly anyone is in county halls on Mondays or Fridays.
"Why are they spending money on DEI? Why are they spending money on climate change initiatives? Why are they spending tens of millions up and down the country on cycle lanes that no one uses?
"I do think there are big reductions that could be made. I do think an American DOGE-style department for every council in the country is needed. Does that solve all of the problems? No. But it's a start."
Reform has vowed to "reindustrialise" Britain and wants to lift the tax-free allowance on earnings to £20,000.
"Reform will reindustrialise Britain and we'll have a proper, sound industrial policy, but it only works if we start producing enough of our own gas and oil and coal, and all of those things," Mr Farage said.
He said the level at which we start paying tax should not be "frozen the way it is year after year after year".
"We believe the most dramatic tax change that is needed is to lift the level at which people start paying tax to £20,000 a year. That will mean people on low pay genuinely will be a bit better off.
"But equally important, it would incentivise those on benefits, who know that if they work for more than 16 hours a week, all their benefits will be taken away. It would incentivise them to get back to work. So I firmly believe that we are the party of working people."
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