If proof were needed of how misguided it is to conduct British politics by trying to please the Sun newspaper and the Murdochs, it came when the Czech president, Václav Klaus, signed the Lisbon Treaty on 3 November. By rejecting David Cameron's appeals and following the verdict of his country's constitutional court, Klaus removed the final hurdle to the treaty becoming law - and made a nonsense of Cameron's September 2007 promise, in an article for the Sun, to hold a referendum on "any EU treaty that emerges from these negotiations".

Now, to the anger of some hardened Eurosceptics in his party, the Conservative leader, for all his rhetoric, must sit by and watch the treaty further streamline Europe-level decision-making. It introduces a smaller European Commission, new posts of president and high representative for foreign affairs, and the removal of national vetoes in a number of policy areas.

Doubtless Cameron will claim that he did all he could to prevent the treaty from becoming law - even earning the ire of his fellow European conservatives Angela Merkel and Nicolas Sarkozy. But as his arch-Eurosceptic backbencher Bill Cash told me last month, scores of Tory MPs, not to mention PPCs, will carry on fighting for a vote on Lisbon. They will put pressure on their leader by reminding him of his Sun article, in which he promised: "If I become PM, a Conservative government will hold a referendum." With the Czech ratification of the treaty, this "cast-iron guarantee" is looking decidedly dubious.

On the offensive

Yet again, the Tories find themselves in difficulties over Europe - and on multiple fronts. The same questionable judgement that led to Cameron's Sun pronouncement was evident in another of his pledges on Europe - made two years previously to win his party's leadership. That promise was to withdraw from the mainstream European People's Party in Strasbourg. It led Michael Heseltine to break his silence on Europe and call on the Tory leader to rejoin the EPP if he gains power.

Meanwhile, the Foreign Secretary, David Miliband, was able to go on the offensive, denouncing the odd Polish and Latvian members of the Tories' new European Conservatives and Reformists alliance. Friends of the Foreign Secretary say that he has "found his voice" again in recent weeks, starting with his well-received declaration at the Labour party conference in Brighton that the new Tory alliance in Europe "makes me sick". Miliband is once more seen as a serious leadership contender. He is back in the game.

Which is where the treaty also poses an acute dilemma to the Foreign Secretary. With ratification of Lisbon, there is now little doubt that the occupants of the two new EU roles will be decided this month in Berlin, where European leaders gather on Monday for the 20th anniversary of the fall of the Berlin Wall. Miliband is being touted as a candidate for the foreign affairs post.

Miliband says he remains as committed as ever to British politics, insisting that he is "proud to be Foreign Secretary in Gordon's government". At the time of writing, his senior advisers were adamant that he would not take the job if offered it. "David is staying put," I was told. "There really is nothing more to say." Allies believe the Foreign Secretary has much to offer on the domestic scene. "He belongs here," Peter Mandelson has told friends.

However, Miliband would not be the politician he is if he were not tempted by the challenge of such an influential new role, in charge of a small army of diplomats and able to shape, for the first time, a coherent European foreign policy. He has claimed that "I am not a candidate", and "I am not available", but this may have been influenced in part by his inability to campaign while another British candidate, Tony Blair, remained in the running for the role of EU president. (Miliband will only be able to express a desire for the foreign affairs job, should he want it, if Blair pulls out. Otherwise, it will not be offered to him.)

The Foreign Secretary has won unsought plaudits from influential figures in Europe. "He has a good reputation and his standing is good. He is pro-European and constructive," says Martin Shulz, president of the Socialist Group of MEPs. And Denis MacShane, the well-connected former Europe minister, says: "I keep hearing glowing reports in Europe on David. He knows how to talk to Europeans on their own terms." It may also have occurred to Miliband that Peter Mandelson benefited from his four-year stint as the EU trade commissioner, returning to Westminster with his power and reputation enhanced.

Red thread

The difference is that if David Miliband returned to Britain in four or five years' time, his younger brother, Ed Miliband, might then be leading the Labour Party. The Climate Change Secretary, it should be repeated, hates talk of the leadership, refuses publicly to consider it and has banned his aides from doing so. Some of his more zealous supporters have already pointed out that his elder brother's departure would "leave the path clear", thus solving "the problem" of "Ed's possible reluctance to stand against David".

Yet though Ed Miliband, as I predicted here almost a year ago, has emerged as a natural future leader, his elder brother remains the most credible candidate to replace Gordon Brown at this stage - certainly in the unlikely event of Brown stepping down or being forced out before next year's general election. Even his potential rivals for the leadership say that Labour needs David Miliband. "We can't spare him," said the deputy leader, Harriet Harman.

Miliband's prospects at Westminster are sometimes written off because he is seen, crudely, as too "Blairite". But he is more complicated than that. He demonstrated as much in his long interview with the New Statesman in February, arguing for a "red thread of Labour" to run through all policies. "Part of our job is to make sure that red thread isn't lost," he told Jason Cowley, editor of the NS.

This past week, Europe has returned to dominate British politics. Miliband may face a difficult choice, though the likelihood remains that he will not opt for Brussels. Instead, it is the Conservative Party, rather than Labour, that has been damaged by developments in Europe. The proposed Lisbon referendum may be dead, but the old arguments surrounding it are very much alive.