Nigel Farage has said he is going "back on the road" to campaign against the prime minister's Brexit plan.
In the Daily Telegraph, the UKIP MEP said Theresa May's Chequers agreement was a "sell-out" as it included regulatory alignment with the EU.
He wrote he would join pro-Brexit group Leave Means Leave at UK public events.
Meanwhile, ex-civil service head Lord Kerslake has said consequences of a no-deal Brexit would be so serious, MPs would have to reconsider it.
The announcement by former UKIP leader Mr Farage comes after a string of resignations last month over the prime minister's Brexit strategy - including those of David Davis and Boris Johnson.
Mr Davis quit as Brexit secretary saying he did not agree with Mrs May's proposals, while former foreign secretary Mr Johnson accused the prime minister of pursuing a "semi-Brexit".
Mr Farage said "scores of people" had stopped him in the street to ask when he was "coming back".
He added: "Well now you have your answer: I'm back."
The 54-year-old said a "battlebus" had already been hired.
He later told the BBC that Mrs May's proposal was "a complete betrayal of what people voted for".
His comments also come amid calls for a second referendum on the final Brexit deal.
Campaign group People's Vote has also criticised the government's handling of negotiations with the European Union.
People's Vote argues the public should be allowed a say on the final deal agreed with the EU.
Lord Kerslake told BBC Radio 4's Today programme that, if the government was unable to strike a deal with Brussels, there would have to be a "pause" in the Article 50 process. under which the UK will leave the EU on 29 March 2019.
The crossbench peer said in those circumstances, the European Commission would likely insist on some "re-examination" of the 2016 referendum decision to leave.
"The consequences of a no deal would be so serious as I think Parliament would have to seriously consider whether it could contemplate this," he said.
"The question people need ask themselves is: is this a risk that they think we should be taking?
"If the government can negotiate a good deal, then so be it.
"But if they can't and we end up in this position, then we have to reopen the question of whether we go forward with Brexit at all. It is not too late to do that."
The government is due to publish a series of technical notes on preparations for a no-deal Brexit on areas including farming and financial services. But Lord Kerslake said this was "too little, too late".
On Friday, Foreign Secretary Jeremy Hunt insisted Britain would "survive and prosper" if it left the EU without a trade deal - but added it would be a "big mistake for Europe".