Voters will "live with the consequences for years" if they fail to switch their votes to the Tories, George Osborne warned as he said the party would work to address the "anger and anxiety" which produced significant Ukip advances in local elections.
The Chancellor was careful not to criticise people who backed the Eurosceptic party in Thursday's polls, insisting he had "respect" both for them and Ukip leader Nigel Farage, despite him being forced to deny claims of racism over remarks about immigrants.
But dismissing calls for a Tory electoral pact with Ukip, he said the public would be confronted with a clear choice between the two main parties at the 2015 general election and that voting for the minority party could allow Labour to take power.
He spoke out as Labour sought to defend its campaign after results which suggested leader Ed Miliband was not on course to secure a Commons majority despite notching up some notable successes.
Ukip gained more than 120 seats on English councils, eating into the representation of all three main parties and making significant inroads in areas such as Essex seen as pivotal to next year's general election result.
Mr Farage hailed the results as proof that his Eurosceptics were "serious players", with their sights now firmly set on getting MPs elected - and continued to predict that the party would win European elections when the results are declared tomorrow.
But the scale of the "earthquake" predicted by Mr Farage was thrown into question by projections of national vote share which suggested that Ukip support had dropped since last year's general elections from 23% to 17%.
That analysis, by the BBC, also heightened concerns within the Labour Party over its prospects of seizing power at Westminster in 2015 as Mr Miliband came under fire for a disappointing performance.
It put Labour on 31%, only two points ahead of the Conservatives and off course for outright general election victory.
Mr Miliband defended the campaign, which saw the party gain 330 seats, perform strongly in London and notch up a series of significant victories in several target locations.
But it did not secure key councils such as Swindon, was prevented from taking charge in its number two parliamentary target seat, Thurrock, by the Ukip advance, and saw nerves were heightened by strong progress for Mr Farage in traditional Labour heartlands including Rotherham.
Shadow Chancellor Ed Balls said the results were "not good enough yet" and the party had to make arguments about controlling immigration and European reform "more loudly".
Backbenchers rounded on "pointy heads" in London for failing to confront the threat from Ukip more directly and an "unforgivably unprofessional" campaign.
Shadow Cabinet Office minister Michael Dugher conceded the party had to "fight much harder" to tackle the Ukip threat in traditional northern heartlands but insisted the party had made "extremely good progress" and remained "on course for a majority" in the Commons.
Mr Miliband not only "talks about immigration a lot" but had changed Labour policy to deal with the concerns of voters and the party's past mistakes, he said, calling it "one of the defining parts of his leadership".
"We are winning in the areas we need to win," the Barnsley East MP told the BBC Radio 4 Today programme.
Mr Osborne said the predicted post-election "turmoil" in the Tory ranks had turned out to affect Labour instead and played down pressure for a pact.
"What has been interesting over the last couple of days is that many prominent Conservative backbenchers, including those on the right of our party, have gone out there and said we've got to stay disciplined, we've got an effective campaigning organisation now, we've got a good political message," he told Today.
Seizing on Mr Farage's call for voters to use this week's elections to send a message to mainstream parties, he said: "A general election is certainly not a 'free hit' - people live with the consequences for years afterwards and there would be a very big difference between a Labour government and a Conservative government.
"The job of myself and the Conservative Party over the coming year is to really focus people's minds on that choice. We are not remotely complacent about the results on Thursday even though we made some important progress.
"One of the reasons at the last general election we didn't get an overall majority was that lots of people who didn't want Labour voted Liberal Democrat. We have to make sure people are clear about the choice at a general election, clear that only two people can be prime minister."
He went on: "We take very seriously the fact that people voted Ukip, we respect that fact.
"But we have to listen to people who voted Ukip, we have to listen to their anger and their anxiety. We need to respond to that anger and anxiety with answers and we need to deliver on a long-term economic plan that is the answer to any of the challenges facing this country."
Mr Osborne steered away from criticising Ukip, saying he had "a great deal of respect" for those who voted for the party.
Asked if that included the leader, he said: "I respect Nigel Farage and the other leaders of Britain's political parties but I do not agree with him."
Mr Dugher said: "In many parts of the country, Labour spoke to that anger and we were the party of change and we did win those elections. In other parts of country we didn't, which means we do have more work to do and that's what we are going to be doing every week for the next 11 months."
Ukip successes were not surprising when there was "Farage-mania on the telly every night", he added.
In defence of his party leader, he said: "One of his famous phrases is that he's the 'eternal warrior against complacency'."
Mr Osborne told activists and MPs at a post-election conference that the results proved there was "no tide of change" towards Labour, but conceded that voters did not believe Tory promises of an EU referendum.
He said there were "too many people who share our values but did not feel able to vote for us" and said the party would "listen, respond and deliver".
But he was confronted with grassroots concerns that the leadership did not have the answers on the issues that were driving people to Ukip - such as gay marriage, wind farms, overseas aid and immigration.
Speaking at the event, run by the ConservativeHome website, Mr Osborne said: "Labour got 31% of the vote, 8% less than two years ago.
"For an opposition aspiring to win the election next year, Labour had a miserable set of results.
"But the rejection of the Labour Party and the Liberal Democrats must not make us Conservatives remotely complacent.
"There are too many people who share our values but did not feel able to vote for us on Thursday night.
"The results yesterday showed that and I'm sure the results tomorrow will confirm it."
He went on: "This is how we will respond to the vote.
"We're going to do what I believe the public wants us to do: listen, respond, and deliver.
"Listen to the legitimate concerns people have about our economy, about immigration and welfare, about our schools and about Europe.
"Respond to the anger justifiably felt with answers.
"And then deliver with an economic plan that provides the security that families crave."
Facing questions from the floor, however, Mr Osborne was warned that he was not seeking to answer the right questions.
"You are going to listen to Ukip voters and respond to them," one audience member told him.
"But what will you say to them when the Ukip voter says they oppose HS2, they are in favour of grammar schools, they want foreign aid cut, they want EU immigration controlled, they want to scrap subsidies on wind farms and solar panels, they are uncomfortable about gay marriage and they think the Help to Buy scheme is barmy?"
The Chancellor defended his flagship mortgage scheme as a key way of meeting concerns about a housing shortage - facing a shout of "Build more homes" from the floor.
And he said the proposed high-speed rail line would boost jobs and growth, suggesting that Ukip had once been in favour of three such developments.
Mr Osborne conceded that voters remained to be convinced by David Cameron's pledge - aimed at neutering the Ukip challenge - to hold an in/out referendum on EU membership by 2017 if the Conservatives win the 2015 general election.
But he urged people to "stop complaining" about that and join forces to reinforce the message.
"David Cameron could not have been clearer. The Conservatives will not form a Government, he will not be the Prime Minister, unless that referendum is assured.
"We've left ourselves no wriggle room. We need to convince the public of that.
"We need them to understand that the only way that referendum on the EU will be achieved is with the Conservatives in Government. That it requires David Cameron and not Ed Miliband as Prime Minister.
"We should stop complaining that the public doesn't believe us on our referendum pledge, and instead work harder to make sure they do.
"And every Conservative minister, MP, and candidate should help us to do that."