David Cameron has vowed to oppose the installation of Jean-Claude Juncker in the European Union's top job "right up to the end" amid reports that Germany is pushing to have the appointment confirmed.
The Prime Minister said there was an "important principle at stake" about the way senior appointments in Brussels were made and it would be "completely wrong" for him to abandon his position.
He urged other European leaders in favour of reform to "stand up and fight" for change within the EU.
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Mr Juncker has been put forward by the centre-right European People's Party -the largest grouping in the European Parliament following last month's elections -but is regarded in London as an opponent of reform whose appointment would make UK departure from the 28-nation bloc more likely.
Mr Cameron has made clear that he believes EU treaties give the power to nominate candidates for top jobs to national heads of government, meeting in the European Council, and not to the European Parliament.