He reveals the decision publicly in an interview with the Daily Telegraph, who have rather tucked away the revelation, which gets a brief mention near the end of the interview with Mary Riddell and Andrew Porter, on page 18 of Friday's print edition, and not yet online. [UPDATE: Read the full Telegraph interview].
By a stange irony, his closest contender may be his wife. Yvette Cooper, who declined to stand for the leadership because the timing was too difficult, is now her husband's "best supporter and toughest critic".
We ask if it is true that the couple had struck a Granita-style deal allowing him a free run at becoming Shadow Chancellor. 'No, of course not. We haven't talked about it. The one thing you can absolutely guarantee is that Yvette will do very well in the Shadow cabinet elections.
A second rumour claims that friends say he would rather serve under Ed Miliband than his brother, in order to avoid risking a reprise of the Blair/Brown wars. Is that true? Mr Balls replies that no one else speaks for him.
Any such statements are "utter fabricated garbage" and anyway [such sources] are obviously not my friends. They''d know better. The answer is: No, I think that's rubbish."
His ballot paper carries no second preference: "I'm going to support whoever wins"
He also says, of the Shadow Chancellor's post:
"I want Labour to win again. I will back the leader 100 per cent. They'll make their decisions and that is fine by me. I am going to be loyal".
He believes the leader must "pick the best people for the best jobs - the ones for which they are most suited", citing disunity and not winning the economic argument as the reasons why Labour has lost previous elections.
Balls is not preparing for victory. He also tells the Telegraph that he has not written a leadership acceptance speech: "If I need one, I'll have to scramble it", he says ... It was always frustrating that people wanted to make it a two-horse race ... If you're still two goals down in the 85th minute, you don't say 'Hang on, can we not make the match 120 minutes?' You play by the rules. "If I'm not winning at full time, then I haven't won and that's just the way it is".
Balls believes that casting no further preferences, after voting for himself as leader, was the best approach, given that he will want to support whoever is elected. It also reflects a view that the Miliband brothers each have different strengths, and confirms his earlier statements that he would not seek to direct his supporters to act as a block in the election, and that they should think carefully and make up their own minds.
Others who nominated Balls can be found supporting both Milibands with transfers: Geoffrey Robinson, Chris Leslie and Tom Blenkinsop have plumped for David Miliband, while John Healey has joined Tom Watson, Eric Joyce and Teresa Pearce in casting a second preference for Ed Miliband.
This is one final post-campaign nail in the coffin for the theory that Balls would choose to seek to be a "king-maker" in this election. (This blog was sceptical, noting that "the powers of a king-maker are easily exaggerated. Perhaps Ed Balls is not going to put them to the test", though the David Miliband campaign had appeared confident of a "deal" to secure Balls' backing in early September).
Next Left believes that Balls' decision will leave the Miliband brothers with two second preferences each among the five leadership contenders (though their own fraternal transfers are unlikely to be counted in the ballot) when the details MPs' votes are published in full next Wednesday.
Balls' own vote is worth a not neglible 0.33% of the Parliamentary section of the electoral college. If he does not make the final round of the last two candidates himself, this will marginally inflate the value of all other Parliamentary votes. Harriet Harman is also reported to have chosen not to vote, as acting leader. It is widely thought that Gordon Brown will cast a vote, most probably for Ed Miliband rather than Ed Balls, but this has not been publicly confirmed.
Another possible final round abstainer might be Yvette Cooper. Next Left asked the Balls campaign yesterday whether Cooper, who had nominated her husband for leader, had also chosen not to cast a second preference for leader. Laudably, the Balls campaign didn't know, which is probably as it should be. As one the strongest rival contenders for the post of Shadow Chancellor, under either Miliband, Cooper too will have had to decide over the last week whether or not expressing a further leadership preference would make sense for her personally.
Balls' interview today remarks in the Telegraph interview reflect that he can legitimately take the view that any so-called friend who told the New Statesman that making him Shadow Chancellor under David Miliband would repeat the Blair-Brown wars wants shooting).
Friday's Guardian sees Balls himself minimising deficit differences with both Milibands.
Balls, the shadow children's secretary and a strong contender for the shadow chancellorship, insisted that his call to slow the Labour deficit programme "does not put me in a different place to either David or Ed Miliband". He said: "My Bloomberg speech [last month] has been praised by both Milibands and no one thinks we have to stick by the position adopted two years ago. The world has changed and moved on. The American downturn and the European crisis have changed things. I find it bizarre that Nick Clegg argues the crisis makes the case for cuts. It is nonsense and I think he has been very badly advised."
Similarly, Balls tells The Telegraph "Both Milibands have cited the Bloomberg speech as something which really socked it to George Osborne. So in the big fundamental analysis, we're all in the same place. People will want to find divisions but we're smarter than that and we understand the history, and we have gone through this [contest] on a comradely way".
No full tally of MPs' transfer preferences is likely before Saturday's result, though the Left Foot Forward website have continued to monitor developments most closely.
There have been at least some late changes of allegiance. Austin Mitchell, who helped put Andy Burnham in the race wiht a nomination, finally decided on the final day to vote for Ed Miliband. (Declaring this behind the Times paywall on Thursday as the campaigns wrapped up means the blogs and tweeters mostly missed it). Beneath the media radar, Ed Balls' campaign has demonstrated its characteristic stamina in this long campaign, successfully secured two MPs' first preferences - one each from public supporters of Ed and David Miliband - in the last couple of days before voting closed. Their identities are not being publicly revealed until the votes are counted. (The switcher from Ed Miliband, for example, wished to express support particularly for Balls' economic arguments in his Bloomberg speech without intending this to be a public criticism of the leadership credentials of Ed Miliband, who continued to have his second preference, and of whom he has been a vocal public advocate).
I would gauge that this increases the chances of Diane Abbott being eliminated first - unless she can establish a strong advantage over Balls or Burnham among party members or trade unionists - and also of Ed Balls winning what looks a tight race for third place.
So it may be that Ed Balls supporters' transfers which finally determine who crosses the finishing post - though only if and where they have decided to express a preference between Milibands, unlike the man himself.