Labour leader Ed Miliband has launched a broadside at the UK Independence Party, claiming that its policies would "destroy jobs, our health service and basic rights".
Mr Miliband's comments, in an article for the Daily Mirror, amount to his most direct assault yet on Ukip and will be seen by some as an indication of concern that Nigel Farage's party may outpoll Labour to claim first place in elections to the European Parliament on May 22.
A poll last weekend put Ukip on 31% for the Euro-elections, ahead of Labour on 28% with the Tories in third on 19%. Former Miliband adviser Lord Glasman told the Sunday Times that Labour voters are defecting to Ukip because the party has become too "middle class".
But Mr Miliband dismissed Mr Farage's claim to represent the interests of working people.
"Now we have Ukip and Nigel Farage pretending that they are the real champion of hard-working people," wrote the Labour leader.
"This is from a politician who likes to boast he is the only one 'keeping the flame of Thatcherism live'. And the truth is that Ukip's policies towards working people are more Thatcherite than Lady Thatcher herself.
"His party promises higher taxes for working families and huge giveaways for the rich. He wants bankers' bonuses to be bigger, while risking 3.5 million jobs by pulling out of the EU and scrapping basic rights at work like maternity or sick pay. One of his MEPs has even claimed 'the very existence of the NHS stifles competition' and his party wants to impose charges for visiting a GP."
Mr Miliband said: "I have a clear message for Ukip and Mr Farage: you cannot claim to be a party for working people when you would destroy jobs, our health service, and basic rights.
"There is only one party which truly understands the cost-of-living crisis and will take the action required to make hard-working Britain better off - Labour."
Mr Farage has accused the mainstream parties of trying to "browbeat" voters into abandoning Ukip, after the party was accused by a cross-party group of aping far-right tactics and attracting extremists to its cause.
Labour former minister Barbara Roche - who chairs the Migration Matters Trust with a Tory MP and Liberal Democrat peer - said the party was guilty of "a form of 'Euracism'".
"They are deploying the same language and tactics used by openly racist parties like the BNP, but instead of targeting migrants from Africa and Asia they are targeting migrants from within the EU," she said.
But Mr Farage compared the charge to former Prime Minister Gordon Brown's description of a voter as "bigoted" during the 2010 general election campaign.
"I am really sorry that millions of people who have decided to vote Ukip next month now find themselves accused by the political establishment of supporting racism," he said.
"This is like the incident between Gordon Brown and Gillian Duffy as the last general election writ large: this time it is not merely one person being slandered by one establishment party, but huge numbers of decent British people under attack, and all three Westminster parties levelling the charge of racism and bigotry."
Ukip was now "recognised as a threat to the entire establishment" and the three main parties were ganging up and "slinging mud", said the Ukip leader.
Mr Farage's party announced that it has begun action to expel two members, after an internal investigation found evidence of links to far-right groups.
One of the men was discovered to have been a member of the British National Party from 2005-2010, the second to have been a donor to the English Defence League.
A spokesman said: "UKIP is a non-racist, non-sectarian party and we are determined to uphold those values. Part of that process is maintaining vigilance against the possibility of infiltration either on an organised or individual basis by those who do not subscribe to our values."
The men have not been identified by the party as they still have the right of an appeal in writing to Ukip's national executive committee.
A series of controversies over offensive comments made by candidates and supporters and a stark anti-EU immigration poster campaign have failed to dent Ukip's soaring poll ratings which have put it on course to top the national poll.
Would-be councillor Andre Lampitt was suspended hours after featuring in an election broadcast for expressing "repellent" racist and anti-Islamic views on social media.
Another candidate, William Henwood, resigned his membership of Ukip today after the party said it was "mutually agreed this would be the best course". He sparked fury by saying comedian Lenny Henry "should emigrate to a black country".