Gordon Brown is to question the Tories' credentials as a "modern, mainstream" party, saying the image they "present" is undermined by their policies.
Prime Minister Gordon Brown has accused the Tories of trying to "scare" people into accepting a "bleak" future with talk of the record deficit.
He will tell a conference of centre-left European leaders that Conservative policies would endanger the recovery and hurt middle-class families.
In a speech to centre-left European leaders he attacked the response of the "well-financed right wing" to the financial crisis.
With parties' pre-election efforts moving up a gear, Mr Brown will unveil Labour's campaign slogan on Saturday.
He said they used "legitimate concerns about deficits to scare people into accepting a bleak and austere" future.
Tory leader David Cameron has said Labour has "nothing positive to say".
In January ministers borrowed another £4.3bn to plug the hole in UK finances.
'Conservative tactics'
It comes as more than 60 senior economists sign two open letters that back the chancellor's decision to delay government spending cuts until 2011.
In contrast, Mr Cameron - in a series of interviews on Thursday - said his party's message was "overwhelming positive" and argued that the increased number of female and ethnic minority Tory candidates demonstrated how much the party had changed.
The letters in the Financial Times say that any measures to trim the budget deficit this year could risk dragging the country back into recession.
The general election must be held by June although most commentators believe it will take place on 6 May.
On Sunday 20 economists backed the Conservatives' call for cuts this year in a letter to the Sunday Times.
As the pre-election skirmishes continue, Mr Brown is giving a speech to the Progressive Governance Conference - attended by the leaders of Spain, Greece and Norway among others - in London which he is expected to use to attack the Conservatives and other centre-right parties.
"All over the world, the new tactic of the right is to present themselves as moderate and mainstream," he will say. "But when the tough questions are asked of them, the truth is revealed.
People know that no one country can solve terrorism or conflict or poverty or climate change on their own Gordon Brown
"Instead of helping a recovery, their hatred of government action would risk the recovery. Instead of defending ordinary families, they would kick the ladder of opportunity away from ordinary families."
Labour say the Conservatives are isolated in their calls for spending cuts to begin this year, saying this would threaten jobs and the UK's fragile recovery.
For their part, the Conservatives say borrowing figures published on Thursday, showing the budget deficit rose by a further £4.3bn last month, prove that Labour is unable to "get a grip" on the public finances after the recession.
Describing Labour and the centre-left, in general, as the "true internationalists", Mr Brown will also say that the opposition's "obsession with narrow nationalism is totally out of tune with the modern world".
"People know that no one country can solve terrorism or conflict or poverty or climate change on their own," he will argue. "As long as I remain prime minister, Britain will stay firmly in Europe's mainstream....and will resist the attempts of the Conservatives to pull Britain into isolation and irrelevance."
Pre-election rally
The BBC has learned that, on Saturday, Mr Brown will outline the four main themes he hopes will help Labour to a fourth term in government.
At a rally in the Midlands, he is expected to emphasise Labour's commitment to securing the economic recovery, protecting frontline services, protecting future jobs and industries, and "standing up for the many".
But the Conservatives have accused ministers of spending more time attacking them than making their own case to stay in power.
"I would say we are running a very positive, a very invigorating campaign," Mr Cameron said on Thursday.
"I haven't seen any [Labour] posters they have produced that don't have a picture of me on it. They don't seem to have anything positive to say."
Earlier this week, Mr Cameron launched a poster campaign appealing to people who have never voted Tory before, saying his party had changed and now spoke for the "mainstream majority in our country".
