Eddie Barnes - David emerges from the shadow of a Goliath

THE Romans gave Minorca its name, meaning "little one", in direct contrast to its bigger neighbour, Majorca, the "the large one".

Yesterday, as David Miliband arrived at the smaller of the Balearic islands with his wife Louise and their two adopted children for a two-week holiday, he could have been forgiven for congratulating himself on how the little ones can sometimes bring the large ones to their knees.

Seven days ago, Miliband stood as a relatively tiny figure within Britain's political firmament, overshadowed by the faltering yet still huge shadow of Gordon Brown. The cliche is also a truism: a week in politics is a long time. Now, the question being discussed by MPs as they head for their sun-loungers this weekend, is when the "Millipede" takes over.

The origins of the Foreign Secretary's extraordinary coup de theatre last week ("just breathtaking" in the words of one Labour minister) can be traced back to March 2007, in the last days of Tony Blair's days in office. Allies close to Miliband – or more pertinently, MPs and ministers who dreaded the coming of Gordon Brown – were insistent that the fresh-faced 38-year-old should have a tilt himself at the job. But Miliband held back; for which he was rewarded with the Foreign Office. For a few months, while Brown's star rose, Miliband's decision must have seemed wise, but in recent months as Brown's star crashed and burned, so the recriminations – and the pressure – increased.

Those same allies who had pressed him to take on Brown last year now felt vindicated and angry, accusing him of having "dithered". They warned him bluntly that he could not simply hang around and wait for Brown's Government to implode, with some telling him that if he simply waited until Brown lost the next election for his chance for the top job, they would not support him. Having hesitated once, they told him it was up to him to now make amends. "When you look at why a politician like David does something, it's all about those who are pushing him," said one Labour hand last week.

With Brown away on holiday, and Miliband about to go, the Foreign Secretary had a very narrow window. His Guardian article that appeared on Tuesday may simply have appeared to be a statement of values on the surface, but to old Westminster hands the intent behind it could not have been clearer. The omission of the two words "Gordon Brown" in the 1,000-word piece said it all. One minister declared: "If you do something like that you basically have to regurgitate the old line about 'Gordon being the best man to lead us through tough times, blah, blah'. For David not to even mention Brown in the piece was extraordinary." Both his press conference with a bemused Italian foreign secretary that day and his appearance on Radio Two's Jeremy Vine show were long-scheduled events. Another MP said: "The truth is this: you don't do what he did unless you're up to something."

So was it successful? A poll in the Daily Telegraph last Friday appeared to show that Miliband had had little effect. Asked whether they would vote for Labour under Miliband, only 24% say they would – one less than Gordon Brown. The poll appeared to back claims by Downing Street aides last week that those wanting a new leader should think twice before thinking it would be a panacea (even if the survey, undertaken between Tuesday and Thursday of last week was too soon, perhaps, for the Foreign Secretary's sally into the public arena to have truly filtered through).

But Miliband's allies say that the point of his actions last week was not so much to win over the public, but to send a message to his party colleagues. One Blairite who is now fully behind a Miliband prime ministership said: "By doing what he has done now, he has established himself as a player rather than the passive figure he was up till now. Nobody can now say he hasn't got the balls. We knew he had the youth and the charisma. He is regarded as a player internationally. Now he has the politics too to go with it."

In other words, Miliband had offered proof positive to those disgruntled allies who were questioning his mettle that – no doubt about it now – he was up for the task. In doing so, it has laid to rest another issue: if Brown goes, Miliband is now primus inter pares when it comes to a successor.

The question on everyone's lips now is what happens next? Even those within the Miliband circle say they don't know. In the immediate term, with both Miliband and Brown away for between two and three weeks, a phoney war is likely to break out. Factoids will be seized on for their every significance – witness last week's revelation that Miliband had cancelled a trip to India, news which was immediately seen as evidence of his imminent departure from the Foreign Office. By the end of the month, however, one thing is certain to happen. Events.

The first such definite date is the Trades Union Congress in Brighton, starting on September 8. Speculation about Miliband's intent was stoked last weekend at the party's national policy forum when he was seen enjoying drinks with the brethren long after Brown had left for holiday. Of the coming week on the English Channel, one gleeful conspirator remarked: "It'll all be about who is lunching with the general secretary of Unite and who is sitting next to the head of Unison."

Then, or possibly before then, Brown is certain to hold a reshuffle. Following last week's events, the Prime Minister's handling of his upstart Foreign Secretary certain to be the main focus of attention. What should Brown do? Labour MP Bob Marshall-Andrews has declared that Miliband should be sacked for his insolence. The "nuclear option" of firing Miliband has not been discounted. But Miliband, his allies insist, is now unsackable. "He is so obvious a successor that Gordon can't do it," says one, pointing to the damage it would do to Brown's reputation if he "petulantly" flung him to the backbenches. Blairites are already chuckling about this exquisite repetition of recent history. It was, after all, not so long ago that Chancellor Brown had dug out a similar position for himself. One theory, growing in strength, is that Brown will hand Miliband the Treasury, binding him tightly to the Government's fortunes.

And then, starting on September 20, Brown, Miliband and the whole caravan will head to Manchester for the Labour conference. It was at the same conference a year ago that Brownites ran away with the idea of an early election, so giddy were they with the new Prime Minister's popularity. But of the forthcoming event, one source remarked: "It's going to be like the old days when you had Brown's leadership speech, followed by Blair's. Except now it's Miliband."

Brown's allies fear it will now be impossible for the Prime Minister to shake off the leadership speculation. And they fear it will only get worse. The very fact that Miliband has shown himself ready to take the challenge on has now sent disgruntled MPs and Ministers a signal that any act of rebellion will not be futile. A joint letter from MPs urging Brown to put up or shut up may well be circulated. After that, say restive anti-Brownites, it will be up to "the men in grey suits" in the cabinet to tell Brown that enough is enough.

And yet Brown should never be written off. The only way the Prime Minister will quit, say friends, is if his wife Sarah and his family tell him to, for the sake of his own health. Brown is already working on plans for a new economic package – with a windfall tax on energy companies at its centre – to be unveiled soon after he returns from a trip to the Beijing Olympics. But he now knows he faces the toughest test of his premiership by far. September looks seminal, "It's not going to be pretty," says one minister, with some under-statement.