Ed Miliband has accused David Cameron of "presiding over the biggest loss of influence for our country in a generation".
In a speech on foreign policy at Chatham House, Mr Miliband called for an end to "small-minded isolationism".
He said a Labour government would "stand up for Britain" on the world stage.
ITV News' Carl Dinnen said the Labour leader had launched a three-pronged attack on the Prime Minister's foreign policy citing the examples of Libya, Ukraine and the European Union:
- Mr Miliband said it was "wrong" of David Cameron to assume Libyan institutions could evolve on their own
- He claimed the HSBC threat to leave the UK underlined the danger of in-out EU referendum
- The Labour leader said the Ukraine crisis was an "apt symbol" of Britain's isolation and "waning influence"
Labour's shadow Business Secretary has said that HSBC's announcement it may move its headquarters outside of the United Kingdom, highlights the importance of EU membership for the country.
Chuka Umunna said:
HSBC's statement today serves to illustrate how irresponsible it is to play fast and loose with the UK's membership of the EU.
It would be a disaster for our financial services sector and business in general if the UK left the EU. Better to stay in and lead reform.
Chief Secretary to the Treasury Danny Alexander has finally responded to the infamous "no money" note left by his predecessor.
Labour's Liam Byrne famously left a note to his successor after Labour's election defeat in 2010, saying "Dear Chief Secretary, I'm afraid that there is no money. Kind regards and good luck."
Five years after taking over, Mr Alexander sent a letter to Mr Byrne apologising for the late reply but that he had been "busy fixing the economy".
"The deficit halved. Jobs up. Growth up. That's the Liberal Democrat record," the Liberal Democrat minister wrote.
"We won't let you - or the Tories - screw it up!"
Nick Clegg has accused Labour of a "distasteful" attempt to make political capital from the Mediterranean refugee crisis.
It follows a row over a speech by Ed Miliband in which he is due to suggest the deaths of hundreds fleeing North Africa are "in part a direct result" of the government's military intervention in Libya in 2011.
Mr Clegg told BBC Radio 5 Live it was "pretty distasteful to reduce this total human tragedy, hundreds of people dying in the Mediterranean, to a political point-scoring blame game".
But he acknowledged that it was "legitimate to say that things then spiral in directions that you can't fully predict".
"All I would plead for is a little less finger-pointing wisdom from Ed Miliband - when he supported the intervention in the first place - and a little bit more adherence to facts about exactly who is ending up on these boats, why they are and what we can now do about it to stop this terrible tragedy," he said.
The Environment Secretary Liz Truss has said it is "absolutely offensive" for Ed Miliband to imply David Cameron has some role "directly or indirectly" for the recent deaths of migrants in the Mediterranean.
In a foreign policy speech later today, the Labour leader is expected criticise the prime minister for the ”failure of post-conflict planning” after the fall of the Gaddafi regime in Libya.
The ongoing instability in the country has been blamed for the boom of people-trafficking crossing from Libya to the Mediterranean and the subsequent increase in migrant deaths on these boats.
Ms Truss said: "To bring this into an election campaign I think is outrageous and disgraceful.
"Actually accusing the Prime Minister of causing these deaths - whether directly or indirectly - I think is wrong."
"Of course we should be talking about foreign affairs but it has to be done in a proper manner", she added.
The Labour leader has accused David Cameron of presiding over the 'biggest loss of influence for our country in a generation'.Read the full story ›
Labour have blasted Jeremy Hunt after he admitted voters did not believe the Tories over the NHS.
Shadow Health Minister Andy Burnham said: "Jeremy Hunt has finally conceded what everyone knows to be true, you can't trust a word the Tories say on the NHS.
"The reason why they have a credibility problem is because they have a track record of failing to keep NHS promises.
"Last time out, they promised 'no top-down reorganisation' - but then brought forward the biggest in the history of the NHS.
"People won't believe David Cameron on the NHS at this election because they know he hasn't got a clue where the money is coming from. They know you can't fund the NHS on an IOU.
"While Labour has a better, fully funded plan to invest in the NHS, the Tories have a plan for extreme cuts which will see them cut the NHS."
Jeremy Hunt has compared modern politics to dating app Tinder and called for a return to old-fashioned doorstep canvassing.Read the full story ›
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.