In a foreign policy address ahead of the May 7 vote, the Labour party leader said Conservative Cameron had "stepped away from the world" and was risking the national interest by promising a referendum on Britain's European Union membership.
"David Cameron has presided over the biggest loss of influence for our country in a generation," Miliband told the audience at think tank Chatham House in London.
"This government's approach has weakened Britain at a time when the challenges are perhaps greater and more complex than at any time since the Second World War."
Foreign policy has been little debated in the campaign aside from references to immigration, something the Labour Party said showed the "growing insularity of British politics".
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Miliband described Cameron's absence from peace talks on the Ukraine crisis between French, German, Russian and Ukrainian leaders as an "apt symbol of Britain's isolation and waning influence".
But the Conservatives reacted angrily when Miliband appeared to partly blame Cameron's policy on Libya for the Mediterranean boat tragedies.
In his speech, Miliband said that "Cameron was wrong to assume that Libya's institutions could be left to evolve and transform on their own" following military action to oust former leader Moamer Kadhafi, but did not directly link it to the migrants' plight.
A pre-speech press release, however, said Miliband "will say 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".
Cameron called the comments "ill-judged" while former Conservative foreign secretary William Hague accused the opposition leader of "opportunistic taking advantage of human tragedy in order to serve the electoral purposes of the Labour Party".