David Cameron has insisted he can "do business" with Jean-Claude Juncker just days after denouncing the arch-federalist's nomination for the top job in Brussels as a "bad day for Europe".
In what will have been a difficult telephone call to make, the Prime Minister congratulated the former Luxembourg premier on securing the support needed to become the next president of the European Commission.
Mr Juncker said he was "fully committed" to finding a solution to British concerns about the European Union during the call and they discussed "how they would work together" to boost competitiveness, according to No 10.
The Prime Minister has faced criticism of his "cack-handed" negotiating tactics after failing in his bid to block Mr Juncker's nomination to the crucial post.
He will be quizzed in Commons tomorrow over the appointment but Tories have insisted he is set to receive "amazing support" from his backbenchers over the position he took.
Labour, however, have branded the failed negotiations as a "catastrophe" for Britain that have taken the UK closer to the "exit door" that could put three million jobs and tens of thousands of businesses at risk.
Writing in The Daily Telegraph, Mr Cameron has insisted he can still secure reforms to Britain's relationship with Europe.
"I am ready to move on and keep fighting for Britain's interests in Europe," he wrote.
Referring to previous claims by Mr Juncker that he was ready to deliver a "fair deal" for Britain, Mr Cameron said: "If by a fair deal, we can agree that we are not heading, at different speeds, to the same place - as some have assumed up to now - then there is business we can do."
Mr Cameron said his decision to stick to his word showed he meant business.
He added: "Anyone in Europe who thought I was going to back down or blink is now thinking again."
A Downing Street spokesman said: "The Prime Minister called the Commission President-designate, Jean-Claude Juncker, this afternoon. The Prime Minister congratulated Mr Juncker on running a successful campaign and securing the Council nomination.
"They discussed how they would work together to make the EU more competitive and more flexible.
"The PM welcomed Mr Juncker's commitment of finding a fair deal for Britain and Mr Juncker said that he was fully committed to finding solutions for the political concerns of the UK.
"The PM wished Mr Juncker well with the hearings in the European Parliament and they agreed to speak further at the next European Council on 16 July.
Shadow chancellor Ed Balls said he had "never seen a negotiation so cack-handed".
"We won't be influential in the world, unless we are influential in Europe," he told BBC One's Andrew Marr Show.
"What is David Cameron's European policy? Does he think we should be in Europe? I do. I think that we will only be influential in the world if we are influential in Europe. Does he want reform? He says he does but he can't tell us what his reforms are.
"He's set an arbitrary timetable for a referendum which everybody knows is deeply destabilising and why has he done that, because his European policy is not being decided by the national interest, Britain's future, jobs and investment, but by what Tory backbenchers are demanding.
"He's weak, he's lost control, he's on the back foot, Britain is suffering. I think it's catastrophic for Britain."