He doesn't open up very often, but past form suggests than when the reserved former chancellor Alistair Darling chooses to speak out, there's no holding him back. Which is why former Labour ministers are likely to anxiously run their finger down the index when his memoir is published next week.

Three years after bluntly – and it turned out rightly – declaring in September 2008 that Britain was facing "arguably the worst" economic downturn in 60 years, the sober Scot has spent some of the free time on his hands since returning to the backbenches penning a memoir in which he reportedly reveals the true scale of behind the scenes feuding that dominated the Labour government during the financial crash.

Back from the Brink: 1,000 days at No 11, charts the breakdown of his relationship with Brown and accuses the former prime minister of attempting to undermine him at the Treasury, according to Labour Uncut, a Labour-supporting website which claims to have seen the book ahead of publication.

The book also reportedly describes the Bank of England governor, Mervyn King, as "amazingly stubborn and exasperating" as Darling lays bare tensions at the top of government and the Bank of England during the 2008 financial crisis.

Darling apparently confirms, as was widely rumoured at the time, that Brown did indeed try to sack him in 2009 and offer him another role in cabinet.

But the chancellor refused, making clear he would rather walk out of government than do any other job. His calculation that the beleaguered premier's weak leadership could ill afford to lose a chancellor turned out to be right, and Darling stayed put.

Darling describes Brown as having an increasingly "brutal and volcanic" demeanour, mistrusting his chancellor to the extent that he repeatedly tried to place his own aides in the Treasury ministerial team to report back on what the chancellor was doing.

He is reported to single out Ed Balls, now shadow chancellor, and former business minister Shriti Vadera who, as key allies of the former prime minister, were running what amounted to a parallel Treasury operation within government at the time.

Atlantic Books, which is publishing the diaries, declined to comment.

The Conservatives, meanwhile, wasted little time seizing on the reported references to Balls.

"Alistair Darling's memoirs should give Ed Miliband some concerns about Ed Balls's suitability to be shadow chancellor," said Conservative party chair Sayeeda Warsi.

"Ed Balls recently claimed that he 'did his politics on the record', but he has already been shown to have been at the heart of the plot to oust Tony Blair. Now Alistair Darling accuses him of running a shadow Treasury operation within his own government. No wonder Labour left the nation's finances in such a mess when they put party political plotting above the national interest."

A Sunday newspaper has been signed up to serialise extracts this weekend, ahead of the book's publication next Wednesday, 7 September.