David Cameron reportedly threatened to take Britain out of the European Union if its leaders back Jean-Claude Juncker as the next European Commission president, it has emerged.
The prime minister made the threat in last Tuesday's Brussels summit where EU leaders gathered in the wake of elections to the European parliament, which saw a huge increase in the number of anti-establishment eurosceptic MEPs.
No 10 has denied Cameron told Angela Merkel that "a face from the 1980s cannot solve the problems of the next five years", as was quoted by the German magazine Der Spiegel.
It suggested Cameron warned his government would be so destabilised by a Juncker presidency the in-out referendum would have to be brought forward.
Juncker, a notorious federalist, is backed by the Christian Democrat and socialist families across Europe, but Cameron is working hard to undermine his bid to replace Jose Manuel Barroso.
He could find an ally in French president Francois Hollande, who is reportedly hoping a French official can get the key job.
Cameron's aggressive negotiating strategy may in part be influenced by the pressure he faces from Cabinet colleagues like Iain Duncan Smith, who revealed he expects Cameron to become increasingly robust in his demands.
"I think you will find that the prime minister in due course will be a little clearer about some of the stuff that he thinks," the work and pensions secretary told the Sunday Telegraph newspaper while on the campaign trail in Newark.
"The key thing is he knows very well that he can't come back with nothing.
"I don't think the prime minister in any way is lost on that fact that he will need to get back some substantial and significant return of powers. It is for him to deliver that."
Conservative activists and MPs have made clear their concerns about the limited progress of Cameron's 'renegotiation', which the prime minister hopes will precede the in-out referendum to be held no later than 2017.
Shadow foreign secretary Douglas Alexander has sought to intensify the pressure by writing to Cameron with a demand for him to provide the details of his reform agenda.
"It is high time you shared with the British people your proposals to change the EU," Alexander wrote.
"It is undeniable that Europe has undergone an economic and financial crisis, but the priority now must be not to let this also become a crisis of political leadership."
Labour wants Britain to demand:
- The appointment of an EU growth commissioner
- Extend transitional arrangements on freedom of movement for new members of the EU
- Protect the integrity of the British benefits system
- Review the EU budget
- Introduce a red-card mechanism giving national parliaments the power to block EU legislation