German magazine Spiegel said David Cameron has warned German Chancellor Angela Merkel he may have to bring the UK referendum on EU membership forward if Jean-Claude Juncker becomes the European Commission chief.
The Prime Minister said he sees Mr Jucker, Mrs Merkel's candidate for the post, as too federalist and likely to damage his hopes of reforming Britain's EU ties.
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Prime Minister insists the UK will maintain a strong relationship with Europe especially on trade and security, even after it leaves the EU.