Labour must promise another Brexit referendum to counter the electoral challenge posed by Nigel Farage, the party’s deputy leader has said.
Writing in the Observer, Tom Watson said his party could not “sit on the fence” about the biggest issue to face the UK for a generation.
But ex-UKIP leader Mr Farage said a new referendum would be “a total insult” to five million Labour Leave voters.
The UK has been given an extension to the Brexit process until 31 October.
This means the UK is likely to hold European Parliament elections on 23 May.
Mr Farage launched his new Brexit Party last week and said it had a list of 70 candidates to fight the May elections.
Mr Watson warned that Labour would not defeat Mr 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,” he added.
Voters ‘deserve better’
He said a “confirmatory” referendum and “final say” on any deal was “the very least” voters deserved, now they knew more about what Brexit would mean.
He added: “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.”
However, Mr Farage accused Mr Watson of breaking promises to the British people and said he intended to “wholeheartedly target Labour lies and dishonesty in the weeks ahead”.
Mr Farage’s Brexit Party also poses a threat to the Conservative Party, according to a survey for the Mail on Sunday.
The Survation poll of 781 Conservative councillors found that 40% were planning to back the Brexit Party at the May European elections.
Just over half – 52% – said they would vote for their own party. If Brexiteer Boris Johnson was prime minister this figure would rise to 65%, the survey found.
Some 15% said they believed Mr Farage would be the best leader of the Conservative Party – only Mr Johnson was ahead of him, on 19%.
Although Theresa May has said she still wants the UK to leave the EU as soon as possible, she is yet to get her withdrawal deal – which has been rejected three times by MPs – approved by Parliament.
Cross-party talks between the government and the Labour Party are continuing, to find a way through the impasse.
Labour wants a new permanent customs union with the EU, which would allow tariff-free trade in goods.
The government has repeatedly ruled out remaining in the EU’s customs union, arguing it would prevent the UK from setting its own trade policy.
The EU has said the UK must hold elections to the European Parliament in May or leave on 1 June without a deal.