Cameron maintains opposition to Juncker

The spokesman made clear that Mr Cameron will be making the argument that decisions made by the EU – including the choice of nominees for top jobs – could have an impact on the way Britain votes in the in/out referendum he has promised for 2017.

But he dismissed speculation that Mr Juncker’s appointment could result in the referendum being brought forward, insisting the 2017 timetable “will not change”.

Herman Van Rompuy, president of the European Council, sees “no alternative” to the arch-federalist becoming the new president of the European Commission after holding talks with German chancellor Angela Merkel and the “die is cast”, according to unconfirmed Press reports.

Mrs Merkel wants “to proceed as soon as possible with the appointment” and diplomats have warned that a “battle royal is coming”, the reports say.

Mr Cameron and Mrs Merkel have been at loggerheads over the choice of a successor to Jose Manuel Barroso. 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 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.