Labour's deputy leader has blasted his own party for what he calls "mealy-mouthed" support for a second Brexit referendum.
Tom Watson demanded Labour stop "sitting on the fence" and swing fully behind a public vote in order to defeat Nigel Farage.
His explosive intervention will heap new pressure on Jeremy Corbyn - who has stopped short of backing a "confirmatory" vote on any Brexit deal.
Instead Labour is choosing to keep a second referendum as just one option on the table, despite a Shadow Cabinet split over whether to go further.
In Jeremy Corbyn is not worried by Nigel Farage and is confident ahead of elections Mr Corbyn said "we're not worried about" Mr Farage, whose new Brexit Party has enjoyed a surge in the polls.
But the ex-UKIP leader has today boasted his Brexit Party has received £1.5million in small donations - and said his "priority" is to tour Labour heartlands to scoop up support.
Writing in the Observer, Mr Watson said Labour must combat Farage with a "radical alternative" to plot a way forward out of the Brexit "crisis".
Jeremy Corbyn is not worried by Nigel Farage and is confident ahead of elections
He wrote: "Labour won’t defeat Farage by being mealy-mouthed and sounding as if we half agree with him.
"We won’t beat him unless we can inspire the millions crying out for a different direction.
"We won’t win if we sit on the fence about the most crucial issue that has faced our country for a generation.
"One poll published last week shows we can cut Farage’s lead to 3% if we back a new public vote on Brexit – but that his margin of victory would extend to 10% if we don’t."
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He added: "Now that we know a bit more about what Brexit means, the very least that Leavers and Remainers deserve is a final say - a confirmatory referendum - on any deal.
"They deserve a Labour party that offers clarity on this issue, as well as the radical vision for a new political economy achieved by working with our socialist allies inside the EU.
"And, above all, they deserve better than Nigel Farage's promise of a far-right Brexit that would solve nothing."
Mr Farage accused Mr Watson of breaking his promise to the British people, claiming a second referendum would be "a total insult" to five million Labour Leave voters.
"I now intend to wholeheartedly target Labour lies and dishonesty in the weeks ahead," he warned.
The momentum generated by Mr Farage's Brexit Party has also sent shockwaves through the Conservative Party.
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A survey of 781 Conservative councillors for the Mail On Sunday found 40% were planning to back the Brexit Party at the May 23 European elections.
Just over half - 52% - said they would vote Tory at the European election, a figure that would rise to 65% if Theresa May was replaced by Brexiteer Boris Johnson, the Survation poll for the Mail On Sunday found.
Some 15% of those surveyed said they believed Mr Farage would be the best leader of the Conservative Party - only Mr Johnson was ahead of him on 19%.
