Nick Clegg has accused Labour of a "distasteful" attempt to make political capital from the Mediterranean refugee crisis.
The Liberal Democrat joined 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 coalition Government's 2011 military intervention against Libyan dictator Muammar Gaddafi.
Mr Miliband is due to tell the Chatham House think-tank, the day after Prime Minister David Cameron committed one of the UK's biggest warships to the mission to end the death toll at an emergency EU summit: "The tragedy is that this could have been anticipated.
"Since the action, the failure of post-conflict planning has become obvious. David Cameron was wrong to assume that Libya's political culture and institutions could be left to evolve and transform on their own.
"It should have been avoided. And Britain could have played its part in ensuring the international community stood by the people of Libya in practice, rather than standing behind the unfounded hopes of potential progress only in principle."
While extracts of his speech did not make a direct link, the party made clear that his message was that "the refugee crisis and tragic scenes this week in the Mediterranean are in part a direct result of the failure of post-conflict planning for Libya".
Environment Secretary Liz Truss said Mr Miliband should withdraw the "outrageous and disgraceful" suggestion of a link.
"Accusing the Prime Minister of causing these deaths - whether directly or indirectly - I think is wrong," she told BBC Radio 4's Today.
"Ed Miliband feels like he's losing the argument and he's lashing out. Of course we should be talking about foreign affairs but it has to be done in a proper manner."
And Mr Clegg said it was "pretty distasteful to reduce this total human tragedy, hundreds of people dying in the Mediterranean, to a political point-scoring blame game".
"Particularly as we are now bringing politics into this, from the party that of course brought us into an illegal invasion of Iraq for which there was no planning at all for the aftermath," he told BBC Radio 5 Live.
"Labour supported the invasion. It is very easy to be wise with hindsight."
Mr Clegg insisted that "a considerable amount of thought went in by the international community" to the aftermath and "a huge amount of help and assistance and money has gone into trying to do our bit from outside".
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.
Shadow foreign secretary Douglas Alexander said the Conservatives were manufacturing a row, accusing Mr Cameron of having "waded in and then walked away" from Libya.
There was "a range of different accounts, explanations and responsibilities" for the crisis, he told BBC News.
"But it is also simply a fact that the reason they are using Libya as that departure point is it is effectively ungoverned space and one of the consequences of the conflict has been the ungoverned space has emerged as a result of the failure of post-conflict planning."
Tory sources said Labour was "clearly rattled by two very bad polls last night".
The surveys gave Conservatives a four-point lead over Labour - the kind of advantage which could leave Mr Cameron with 20 to 25 more MPs than Mr Miliband but short of a majority in the House of Commons.
Mr Cameron is returning to the push for votes at home after announcing the deployment of one of Britain's biggest warships as part of beefed-up efforts to prevent more refugees packing on to tiny boats.
Around 800 died last weekend in the biggest single capsize incident of a series this year that has seen some 1,750 lose their lives as an estimated 35,000 have made the crossing to Europe.
He has insisted, as discussions continue about demands for EU states to take a fairer share of the migrants, that the UK would not be expected to offer asylum to those the Royal Navy helps rescue.
In a campaign stop in the key Midlands battleground, he will step up his efforts to woo Ukip supporters with a warning that a Labour government supported by the SNP would allow "a return to uncontrolled immigration".
And he will also unveil a Tory "English Manifesto" promising separate votes on England-only matters in Parliament, including matters such as income tax levels being devolved to the Scottish government.
In his Chatham House speech, Mr Miliband is due to take aim at Mr Cameron's European policy, with a warning that "small-minded isolationism" from the Tories had led to the biggest loss of British influence abroad in a generation.
In stark contrast to the Conservative promise of an in/out referendum on Britain's membership of the European Union, Mr Miliband will also say he "will never put our national interest at risk by threatening to leave".
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