Brexit Party leader Nigel Farage has warned Prime Minister Boris Johnson not to return from Brussels with Theresa May's "reheated deal".
At a rally in London, he said the Tories will "lose votes to us" in "huge numbers" when voters "realise nothing has changed" if they keep that deal.
He then criticised Labour's "policy of uncontrolled mass immigration".
The party also unveiled some policy plans, but little detail, including scrapping HS2 and inheritance tax.
The UK is due to leave the EU on 31 October.
Mrs May's deal was rejected three times by MPs, with the Irish backstop - a policy Mr Johnson has said he wants to scrap - proving a major sticking point.
The policy is aimed at preventing the return of a hard border on the island of Ireland after Brexit.
Mr Farage told the crowd of supporters, referring to the prime minister and his senior adviser: "Mr Johnson, Mr Cummings - if you do get this through, that you can sell this as Brexit, you're in for a big surprise.
"The British people won't swallow it if they realise nothing has changed, they will not put up with it and you will lose votes to us in absolutely huge numbers - heed that warning please."
He reiterated his pledge that if Mr Johnson campaigns in a general election "for a clean-break Brexit" then "far from fighting against him we will work with him with a non-aggression pact" - something which has already been rejected by Number 10.
"To get this done, we will always put country before party," he said.
Mr Farage went on to criticise Labour for changing its policy on freedom of movement.
The Labour Party's 2017 manifesto vowed to end free movement when the UK leaves the European Union, but delegates at the party conference last week voted overwhelmingly to reject that.
Mr Farage said Labour had decided "to embark upon a policy of uncontrolled mass immigration into Britain".
"All of us in this party recognise that immigration can be a very good and a very positive thing for our nation, but you have to control it sensibly and selectively," he said.
He said the Brexit Party would be the "main challenger" to the Labour Party in "many traditional parts of this country, seats they've held for 100 years".
The rally was the final date of the party's "we are ready" conference tour, which took place in 10 venues across England and Wales and featured some of the party's prospective parliamentary candidates.
The party says it has pledged to invest £200bn in transport and digital infrastructure outside London.
Party chairman Richard Tice also unveiled some policies at the London event without going into detail,
