Nigel Farage says he's ready to lead a "revolution" in British politics, promising he will no longer be "Mr Nice Guy".
He told a crowd of supporters in Coventry: "I said that if I ever did come back into the political fray, next time it would be no more Mr Nice Guy and I mean it. I am angry about what has happened.
"I said many years ago I wanted to cause an earthquake in British politics.
"Now what I am fighting for, and with your support what we will attempt to achieve, is a democratic revolution in British politics."
He insisted his new party was very different from Ukip, saying that he did not want to be a part of a party that promoted "violence, criminal records and thuggery" who engaged in "loutish" behaviour.
And he predicted a shock at the ballot box next month: "I do believe that we can win these European elections and that we can again start to put the fear of God into our Members of Parliament in Westminster.
"They deserve nothing less than that after the way they've treated us over this betrayal."
He told supporters that it was time the "political class" were replaced by people who know how the real world works.
He then revealed Annunziata Rees-Mogg, the sister of Tory Brexiteer Jacob, is to stand as a candidate for the new Brexit Party.
Ms Rees-Mogg said it was a tough decision after years loyally supporting the Tories.
“I joined the Conservative Party in 1984 and this is not a decision I have made lightly – to leave a party for which I have fought at every election since 1987, from Maggie Thatcher through to Theresa May," she said.
“I know which one I’d rather have representing us now.”
Chancellor Philip Hammond
She added: “The point at which our Prime Minister will not listen, not only to her membership, but will not listen to the people of her country… I can’t sit by and let her do it.
“We’ve got to rescue our democracy, we have got to show that the people of this country have a say in how we are run.
“That the politicians are not our masters – they are to do our bidding.”
She will be one of 70 Brexit Party candidates who would be fighting the May 23 elections in England, Scotland and Wales
Questioned on what her brother said when she announced she was changing party, Ms Rees-Mogg said: "He accepted my decision. He knew my mind was made up."
She had not tried to convince him to join in the Brexit Party.
Mr Farage said the party had received donations of more than £750,000 in 10 days, showing that small donors were funding a "rebirth of democracy in this country".
The campaign launch will intensify the pressure on Mrs May, with many Tories fearing they will haemorrhage votes to the new party amid growing frustration with the deadlock in Parliament.
Chancellor Philip Hammond hopes the UK do not have to take part in in any European elections.
"I hope we won't have to hold European Parliament elections. Our fervent aspiration is to get this deal done, so that we can cancel those elections before May 22nd," Mr Hammond told ITV News.
Regarding the new Brexit Party, Mr Hammond is not surprised by Mr Farage's return to politics.
"Nigel Farage will pop up in one guise or another, I'm sure, whenever it suits him but in the end this is a debate about Britain's long-term future."
Voters must register for the elections by May 7 and can apply for a postal vote until May 8. Polling stations will open from 7am to 10pm on May 23.