If Darling goes, Brown is finished.

Following Purnell’s resignation, Downing Street has said:

The Prime Minister is disappointed by the resignation of James Purnell, of which he was informed shortly before 10 pm. His focus over the coming days will be on restructuring the Government on the big challenges facing the country for the future: how we guide the economy through the downturn and strengthen it for the future; how we push ahead with reform of and investment in our public services; and how we renew trust in our democracy and Parliament. He will continue to give his undivided attention to addressing these great challenges facing our country and putting the interests of the British people first and foremost.

So, Brown’s focus is not only on the reshuffle but also on ‘addressing these great challenges facing our country’.  Coffee House has this:

I can reveal that James Purnell was offered education, which he turned down, and decided to resign because he couldn't go on continuing to go out in public and support a PM whom he'd lost faith in months ago.

UPDATE AT 12.30AM: No10 has denied that Purnell was offered education. I haven't seen what technical weasel words they have used, but they are fibbing. I am 100% confident. And I suspect this won't be the last set of fibs that No10 tell if the reshuffle keeps falling apart at this rate.

Proof that Brown wants to give Balls Darling’s job.  Now consider this from the FT, written before Purnell’s resignation was announced:

Alistair Darling has sent out a defiant message that he wants to stay chancellor.

Mr Darling’s team has done nothing to dispel the impression that he would refuse a demotion – possibly to home secretary.

If the chancellor is sacked, he could become a danger to Mr Brown, particularly if there was any attempt to question his own personal reputation over the Westminster expenses scandal.

The chancellor’s supporters are suspicious of the timing of revelations in the Daily Telegraph this week about his ineligible claim for a maintenance charge on his South London flat. The report surfaced weeks after revelations about his “flipping” of second homes. Mr Darling was wrongfooted by the story, which led to him repaying £668. He claimed for the service charge six months in advance but forgot to repay some of the money when he moved into 11 Downing St.

If, following Purnell’s resignation, Brown persists in moving Balls and Darling does resign, that would surely make Brown's position untenable.