Ed Miliband, the leader of the main opposition Labour party, said Cameron had failed to build the alliances in the EU that he would have required to block the path of a man that the premier believes is an arch-federalist and a roadblock to reform.
"I am afraid that after weeks of bluster and spin from David Cameron, this represents a complete humiliation," Miliband told Sky News television.
"He was supposed to be building alliances, but instead he has been burning alliances, that's why he has failed to deliver, and it's the British national interest that loses out as a result of this decision."
"The roots of this are long-standing. David Cameron has not built the alliances in Europe," Miliband said.
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"The reason for that is that they think he is just being pushed around by his (eurosceptic) backbenchers.
"This is a consequence of years of failure by David Cameron.
"It emphasises the need for a European strategy that builds wider alliances, that builds alliances in the national interest."
He said he would press on with his commitment to re-negotiate Britain's EU membership before putting the new terms to a referendum in 2017, providing he wins a general election in Britain to be held in May 2015.
But Nigel Farage, the leader of the anti-EU UK Independence Party (UKIP), speaking before Juncker's nomination was confirmed, poured scorn on the prime minister.
"It's one thing to say you're going to stand up and fight and go to battle, but if you lose, it doesn't do you much good," said Farage, whose party finished top in European Parliament elections in Britain in May.