Labour has fallen below 30% in an opinion poll for the first time in the General Election campaign.
The poll for the Daily Mirror put Ed Miliband's party on 29% - down four points in comparison to a similar poll last week.
Meanwhile the Lib Dems jumped three points to 10% while the Tories dropped a point to 33%.
Ukip added one point to reach 18% - just one point short of their best rating of the election campaign, while the SNP remained unchanged on 4% and Greens gained one point to put them at 4%.
Meanwhile a survey for ITV's Good Morning Britain found just 16% of 1,500 voters polled said they knew who they would be voting for on May 7 while one in five, 21%, said said they were still "completely undecided".
Lord Mandelson said Ed Miliband has "way exceeded my expectations" and hailed the Labour leader's election campaign.
The former strategy chief and cabinet minister - who last month declined to positively endorse the party leader - said Miliband had confounded his critics and found the national "Zeitgeist".
"I started feeling at the end of last week that something had shifted," Mandelson told Channel 4 News.
"That the plates had moved and Labour has captured a Zeitgeist in this campaign and the most striking feature of it is what's happened to the two leaders.
"Miliband has moved forwards. He gained credibility. He's exceeded most people's expectations. I suppose in a sense mine as well. David Cameron on the other hand has moved backwards."
Asked in March whether the party would do better under another leader, the peer had stopped well short of an enthusiastic endorsement of Miliband.
Ed Miliband has told supporters that the IFS assessment of party manifestos showed the Tories were committed to "extreme spending plans".
"They have also confirmed that Britain would be facing the deepest cuts over the next three years of any advance country in the world," the Labour leader said during a visit to Nuneaton in Warwickshire.
Miliband continued: "It is a plan so extreme that far from protecting the NHS they would end up cutting the NHS.
"It is a plan so extreme that it wouldn't mean three years of the good life, it would mean three more years of the hardest of times."
Ed Miliband has unveiled a new poster campaign with Labour's proposals for a "better plan for the NHS".
ITV News Political Correspondent Carl Dinnen reports from Nuneaton:
Ed Balls, Labour’s Shadow Chancellor, has defended his party's budget plans after the Institute for Fiscal Studies said voters were 'in the dark' over the plans of four major parties.
The IFS’ numbers wrongly assume that Labour will get the current budget only into balance. Our manifesto pledge is to get the current budget not only into balance but into surplus as soon as possible in the next Parliament. How big that surplus will be, and how quickly we can achieve that in the next Parliament, will depend on what happens to wages and the economy.
The Tories might be able to make the cuts but the last five years show they will fail to cut the deficit as they claim. They have borrowed £200 billion more than they planned because their
None of the top political parties have provided "anything like full details" on plans to cut the deficit in their manifestos, IFS says.Read the full story ›
None of the main political parties has provided "anything like full details" on plans to cut the deficit in the next Parliament, the Institute for Fiscal Studies has said.
Ed Miliband said his new-found sex symbol status was unlikely to worry the boy band as #milifandom takes over the internet.Read the full story ›
Gordon Brown has warned that Nicola Sturgeon's plans would leave a £7.6 billion hole in Scotland's economy, as he pitched Labour as the country's "party of fairness and social justice".
In a letter to voters, the former Prime Minister claimed both the SNP and the Conservatives would leave the NHS at risk.
He said the nationalists would "scrap the Barnett formula, meaning that our NHS and other public services could only be funded by taxes raised in Scotland."
He also warned that the Conservatives were "committed to billions of pounds more austerity - which means less money for Scotland and our NHS".
Labour would hope to begin to "phase out" GCSEs within the next 10 years, the shadow education secretary has said.
Tristram Hunt said English schools had a "long tail of underachievement" and proposed a 14-19 curriculum to replace GCSEs, as he claimed more pupils were staying in education up to 18.
But he said should he become secretary of state for education in a Labour government post-May 7, he was "not going to touch" reforms made by the Conservatives, which included a shake-up of gradings and coursework.