Jean-Claude Juncker may have been endorsed by Germany for the European Commission top job, but Italy and France have signalled opposition, while Britain has warned about the consequences of choosing the former Luxembourg PM for the post.

A British official on Sunday (1 June) confirmed to Reuters that British PM David Cameron at an EU summit last week told fellow leaders that Juncker was not the right person for the job and that if he is picked, it will be more likely that Britons would vote to leave the EU.

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The official however dismissed reports that Cameron threatened to bring forward a referendum on Britain's EU membership if Juncker became commission President.

German magazine Der Spiegel the same day had reported that Cameron told fellow leaders his government would be destabilised to such an extent if Juncker is picked, that the referendum planned for 2017 would need to take place sooner and that Brits would likely vote to leave the EU.

Der Spiegel also reported that German Chancellor Angela Merkel at that same summit opposed Juncker being appointed as the negotiator with member states and the European Parliament, and that she preferred EU Council chief Herman Van Rompuy to do the job instead.

On Friday, however, Merkel gave Juncker her strongest endorsement so far, saying that she is "now conducting all of my talks in the spirit that Jean-Claude Juncker should become president of the European Commission."

But Cameron is not alone in his anti-Juncker stance. Sweden and Hungary have already spoken openly against him.

And on Sunday, Italy's Prime Minister Matteo Renzi said Juncker has no automatic right to the post.

"Juncker is 'one' name for the Commission, but he is not 'the' name," Renzi said during an economic conference in Trento.

Renzi, who is a Social Democrat, is not part of the European People's Party which put Juncker forward as top candidate for the EU elections.

Another centre-left leader, France's Francois Hollande, is also reportedly against Juncker.

German mass-selling tabloid Bild am Sonntag reported that Hollande is trying to block Juncker and to get the post for his former finance minister, Pierre Moscovici.

Hollande's purported argument is that with the far-right National Front winning the EU elections in France, a signal of support for his government is badly needed.

For Juncker to get the nomination of EU leaders for the commission job, a 'qualified majority' is needed, where bigger states have a weightier vote. If Italy and France abstain or say No, while Britain, Sweden and Hungary vote No, Juncker does not have enough votes.

For his part, Juncker remains upbeat. He told Bild am Sonntag he is "optimistic about being chosen as the next Commission president by mid-July".

Being endorsed by EU leaders is only part of the process. He would then also need majority backing in the EP. The parliament is also due to vote again in autumn on the composition of the new EU commission, after conducting hearings of each commissioner candidate.