Tory hopes that the party's election fortunes could benefit from a "Thatcher bounce" after her death have been thwarted by a poll showing Labour has extended its lead.
A YouGov poll for The Sun put Labour 14 points ahead on 42 percent of the vote, a two-point rise on last month, with the Tories trailing on 28 percent, down from 31 percent.
The Lib Dems narrowly regained third place from Ukip, polling 12 percent to the euro-sceptic party's 11 percent, in a reversal of fortunes from last month's figures.
The poll came as Tony Blair warned that Labour's opposition to austerity and welfare cuts risked playing in to the hands of its enemies.
In an apparent rebuke of Ed Miliband, Blair urged Labour not to let itself to be portrayed as the party of knee-jerk protest, as it was throughout the Thatcher years.
Writing in the special 100th anniversary edition of New Statesman, the former prime minister flatly rejected the argument that New Labour was to blame for the financial crisis, insisting the structural deficit had been below 1 percent in 2007-8.
He continued: "The paradox of the financial crisis is that, despite being widely held to have been caused by underregulated markets, it has not brought a decisive shift to the left.
Familiar left-right battle
"But what might happen is that the left believes such a shift has occurred and behaves accordingly. The risk, which is highly visible here in Britain, is that the country returns to a familiar left-right battle. The familiarity is because such a contest dominated the 20th century. The risk is because in the 21st century such a contest debilitates rather than advances the nation. This is at present crystallising around debates over austerity, welfare, immigration and Europe."
Blair said that the Tory party "is back clothing itself in the mantle of fiscal responsibility, buttressed by moves against 'benefit scroungers', immigrants squeezing out British workers and - of course - Labour profligacy." In contrast, Labour has returned to opposing Tory cuts, and highlighting the "cruel consequences of the Conservative policies on welfare and representing the disadvantaged and vulnerable".
He said that this scenario is actually "less menacing than it seems" for the Tories. "They are now going to inspire loathing on the left. But they're used to that. They're back on the old territory of harsh reality, tough decisions, piercing the supposed veil of idealistic fantasy that prevents the left from governing sensibly.
"For Labour, the opposite is true. This scenario is more menacing than it seems. The ease with which it can settle back into its old territory of defending the status quo, allying itself, even anchoring itself, to the interests that will passionately and often justly oppose what the government is doing, is so apparently rewarding, that the exercise of political will lies not in going there, but in resisting the temptation to go there."
He called on Miliband to guard against allowing the party to become a "fellow-traveller in sympathy" by simply opposing Tory cuts, and warned him not to let Labour be cast simply as the "repository for people's anger".
The party needed to be "dispassionate even when the issues arouse great passion", he said.
People want leadership
Blair's remarks will heap pressure on Miliband following warnings of a Labour split over welfare policy. The party was "behind the curve" on the benefits debate, and its leadership had "ducked the issue" too many times, according to several Labour MPs.
Miliband has opposed cuts including the £26,000 a year cap on welfare per family, the below-inflation increase in Job Seekers Allowance, and the "bedroom tax" cutting housing benefit for families with a spare bedroom.
In a passage seen as critical of Miliband, Blair said the public wanted to "know where we're coming from because that is a clue as to where we would go, if elected", and added: "In these times, above all, people want leadership.
"The issue isn't, and hasn't been for at least 50 years, whether we believe in social justice. The issue is how progressive politics fulfils that mission as times, conditions and objective realities change around us.
"It means, for example, that we don't tack right on immigration and Europe, and tack left on tax and spending. It keeps us out of our comfort zone but on a centre ground that is ultimately both more satisfying and more productive for party and country."