Nigel Farage has said Reform UK is becoming a "brand new Conservative movement" and attacked a "lack of debate" between the two main parties.
Speaking in Kent, Mr Farage said there was "no difference" between Labour and the Conservatives over immigration policy.
He defended describing people coming to the UK across the Channel on small boats as an "invasion", saying that the numbers were like a "slow motion D-Day in reverse".
He also expressed concern that the UK was moving towards a "sectarian politics with women completely excluded".
The Brexit campaigner said he was "proud" of the UK's record on integration but said that there was a "growing number of young men" adopting radical views that are "extremely anti-British".
He denied he was Islamophobic saying: "The most worried group about what is going on with this new form of British sectarianism are British Muslims.
"Paying their taxes, working, wanting their kids to do well – they will be worst affected by this current unpleasant trend."
Reform UK – previously known as the Brexit Party – has said it will stand in 630 seats out of a possible 650 across the country.
Mr Farage has ruled out standing as a candidate and hinted that he wants to focus on helping his friend Donald Trump in his bid to win the US presidential election.
However, he has said he still wants to support the party he helped set up and on Tuesday morning gave a speech in Dover, in which he launched a fierce attack on the Conservatives and Labour.
He said Sir Keir had "no energy, no optimism, no substantive policy" and compared Mr Sunak to a "frightened rabbit".
Asked if he was trying to destroy the Conservatives, he replied: "They have destroyed themselves.
"Is there a Conservative Party? I haven't spotted it – all I can see is two big social democratic parties.
"Given that Labour are going to win, why not vote for something you actually believe in."
On Monday, Lucy Allan, who is standing down as a Conservative MP, urged voters in her former constituency of Telford to back the Reform candidate.
Mr Farage added that the election had got off to the "dullest start" he could remember.
The party's honorary president acknowledged Reform UK was "not very well-funded" but said it was offering a "distinct" message.
Asked on Monday if he felt there was a lack of enthusiasm for his party, Sir Keir said recent election results proved that people were turning to Labour.
"I do think there is an underlying issue that over the last 14 years, the government has beaten the hope out of people.
"I want to restore the idea that politics can be a force for good."
Responding to Mr Farage's comments, Mr Sunak said: "On 5 July one of two people are going to be prime minister – Keir Starmer or me.
"The question for voters is on the issues they care about whether its illegal migration or a more sensible approach to net zero that prioritises people’s bills and our energy security, or cutting taxes and rewarding hard work, on all of those issues we are the ones who are prepared to take bold action."
Much of Mr Farage's speech focused on immigration and what he described as a "disturbing trend" of sectarianism in British politics.
Over the weekend he faced criticism when he told Sky News: “We have a growing number of young people in this country who do not subscribe to British values, in fact loathe much of what we stand for."
Asked if he was referring to Muslims, Mr Farage said: "We are.”
He said he was not attacking the religion of Islam but drawing attention to a “real, real problem”.
Liberal Democrat deputy leader Daisy Cooper said: “This a grubby attempt to divide our communities in a desperate attempt for attention.”
Wales is governed both by the Welsh and UK government – meaning only certain issues within the country are on the table this general election.
The Gloucester resident who witnessed the incident on Monday says he thought "it was a bit cheeky".
Sir Ed Davey says his party would put environmental experts on water company boards.
Jesse Norman has been the MP for Hereford & South Herefordshire since 2010.
Martin Geissler, Kirsty Wark and Laura Miller will lead the corporation's election night coverage.
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