Tony Blair has moved to unite the left against Ukip in his strongest attack yet against Nigel Farage and the eurosceptic party.
The former prime minister, who is still reviled by many left-wingers over his decision to invade Iraq in 2003, said Ukip did "not have all the answers" and should be exposed for having "no actual solutions to the problems of the 21st century".
Blair, 61, told BBC Radio 4's Today programme that blaming immigrants for problems is a "backwards and regressive step".
"You look underneath that UKIP facade and you see something pretty nasty and unpleasant," he said.
He was speaking after Ukip topped the European elections on Sunday night, prompting concerns on the left that the main parties would start talking tough on immigration to win back voters who deserted them for Nigel Farage.
The former PM, who served in Downing Street between 1997 and 2007, said: "We are confronted by what I think are these very reactionary forces. We have to confront them, expose them and take them on... you have got to take on and expose the fact these parties have no actual solutions to the problem of the 21st century."
Labour leader Ed Miliband, who has apologised for his party's pro-immigration stance while Blair was in office, has pledged to hold an in/out EU referendum if any new powers are transferred to Brussels.
He has been resisting pressure from within the party to toughen his stance and commit to holding a referendum in the next parliament no matter what.
When asked what Miliband should do, Blair said: "I would advise him to stay firm.
"It is not as if yielding to that pressure from Ukip has actually done the Conservative Party any good at the present time.
"For the Labour Party, if it decides to follow Ukip either on its anti-Europe platform or even worse franky on its anti-immigration platform then all that will happen is that it will confuse its own supporters and it won't actually draw any greater support."
He added: "The way to deal with Ukip is to stand up and take them on. What they are putting before people is a set of solutions that anybody, who analyses where a country like Britain needs to be, knows are regressive, reactionary solutions that would make the problems of the country worse not better."
The interview, broadcast on Tuesday morning, earned the former premier the support of prominent left-wingers, from Labour MP Dianne Abbott to columnist Owen Jones, who wrote in the Guardian that Labour was "doomed" if it tried to compete with Ukip by employing "the politics of fear".
When asked whether he had made mistakes on immigration while in office, Blair said he "very strongly supported the position we took".
Also on Tuesday, two of Blair's key cabinet allies - former health secretary Alan Milburn and former defence secretary John Hutton - wrote in The Times (£) that parties must stop "kowtowing to Ukip".
"Their response so far has been a promise to listen more to public concerns — code for more stringent immigration policy," they wrote, in an article that appeared to have been timed to co-incide with Blair's Today Programme interview.
"Such an approach has serious consequences for the wellbeing of both our economy and our society. Nor will it work politically.
"Every time David Cameron, Nick Clegg and Ed Miliband give an inch on the issue, Nigel Farage takes a mile. Instead of kowtowing to Ukip’s unpleasant form of populism, now is the moment for all three main parties to change tack and make a positive argument for the benefits immigration brings to Britain."
But there is a question mark over whether the party that Blair led for 13 years will listen.
Labour declined to comment on the former PM's remarks when invited to do so by The Huffington Post UK - but Miliband is due to give a speech at 2pm, where he is likely to be asked about his predecessor-but-one's comments.
As for wondering what Ukip's rise would have looked like it happened on Blair's watch, HuffPost UK readers need look no further than this 2005 video of a clash between the then Labour prime minister and a then relatively-unknown and junior Ukip MEP, Nigel Farage.
Farage blasted Blair for not curtailing EU legislation, to which the PM replied by calling him a "reactionary".
Blair told him: "You sit with our country's flag, you do not represent our country's interest."