London: David Cameron warned EU leaders that he could no longer guarantee Britain’s membership of the EU and threatened to move forward an in-out referendum if Jean-Claude Juncker was elected president of the European commission, according to Der Spiegel.

In an attempt to democratise the process of selecting the commission president, Juncker is in pole position as the official candidate of the party that won most seats in last month’s European elections — the European People’s party (EPP).

According to the German news magazine’s behind-the-scenes account of last Tuesday’s meeting of European leaders in Brussels, the British prime minister argued that the former Luxembourg PM’s appointment would politicise the European commission. “The European Commission isn’t like a national government, it has to remain impartial,” he said.

During a break in the negotiations on Tuesday, Cameron is said to have made his opposition to Juncker even more explicit, saying: “A face from the 80s cannot solve the problems of the next five years.”

According to Der Spiegel, members present at the discussions interpreted the British PM’s intervention to mean that he would expedite a referendum on Britain’s membership of the European Union. A lead article in the magazine condemns Cameron’s stance as blackmail.

At the time, Cameron’s performance appeared to have had the desired effect, seemingly swaying the German chancellor: Angela Merkel temporarily distanced herself from Juncker and even announced she would join the leaders of Britain, Hungary, the Netherlands and Sweden in order attain a “blocking minority” against Juncker.

Merkel’s attempt to take the momentum out of Juncker’s nomination may have also been motivated by a desire to take into account French president Franois Hollande’s reservations against the Luxembourgeois.

According to Sunday’s edition of the tabloid Bild, Hollande had been pushing for a French politician as EC president, such as his former finance minister Pierre Moscovici.

But the alliance between the British and German leaders was short lived: after considerable pressure from the German media, her Social Democrat coalition partners and the leading candidate himself, Merkel backtracked once more and on Friday announced her endorsement of Juncker after all.

Leading German publications such as Spiegel, Bild and Frankfurter Allgemeine had published editorials condemning Merkel’s wavering course and suggesting that backtracking on the open nomination process in favour of a backroom deal would amount to deceiving the electorate. According to a poll carried out by Der Spiegel ahead of last week’s elections, 78 per cent of Germans believed that the candidate of the biggest bloc in the European parliament should go on to become president of the commission.

Jean-Claude Juncker said in an interview with Bild that a “broad majority” of European leaders had told him they would support his candidacy. Even Alexis Tsipras, the Europea Left’s Greek candidate for the presidency, had on Saturday announced his support for Juncker. In reference to opposition from Britain, Hungary, the Netherlands and Sweden, Juncker said: “Europe won’t allow itself to be blackmailed.”

Negotiations between the European parliament and European council of national leaders are expected to continue throughout June — with the council announcing its candidate at the end of the month and the parliament casting their vote in July.