BRENDON McCullum says the inexorable pressure of testifying against former teammate Chris Cairns ushered his premature retirement from cricket.

Former New Zealand captain McCullum has revealed the toll that being a witness in the 2015 perjury trial of all-rounder Cairns took on him in his newly released book “Declared”.

Cairns was acquitted of the charges, related to alleged match fixing. That finding came near the end New Zealand’s tour of Australia, during which McCullum made a decision to accelerate retirement later that summer.

He had originally planned to play at the subsequent World Twenty20 in India but found his motivation had waned considerably following a heavily scrutinised role as a witness at Southwark Crown Court in London.

“I’ve tried desperately hard to keep my head locked into the very demanding cricket right in front of me, but to be honest, it’s impossible to ignore the fallout from the trial completely, and it’s been chipping away at me like Chinese water torture right through the Australian tour,” McCullum wrote.

The explosive batsman explains exactly how he broke the stunning news of his retirement to Black Caps coach Mike Hesson, which he did after playing a wild innings during a tour match in Perth last year.

“’I’m done, mate,’ I tell him.

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Brendon McCullum during the presentation after his final Test match. Source: Getty Images

“Hess knows I’ve been struggling, but it seems like a sudden and emotional decision. He asks me if I’m sure I want to retire.

“It is quite sudden and emotional, but I’m sure. I had visions of playing the T20 Worlds in early 2016, then giving away the shorter-format stuff for the Black Caps and just playing Tests, but I’ve come to the end of my tether. I just know in my waters I’ve had enough.”

McCullum goes into further detail around the Cairns trial, expressing surprise he proved to be the key prosecution witness.

He lamented how the trial was portrayed in the media as a battle between himself and Cairns - a former friend and player he first regarded as a hero. He admits their relationship is now irreparably damaged, and says he hopes the two never cross paths again.

McCullum was one of three key Crown witness against Cairns, along with disgraced former cricketer Lou Vincent and Vincent’s ex-wife Eleanor Riley. McCullum expressed concern at the summation of Justice Nigel Sweeney, who instructed the jury to be sure of the evidence of at least two witnesses before they could convict Cairns.

McCullum arrives at Southwark Crown Court to give evidence in the Chris Cairns trial. Source: Getty Images

Vincent was discredited by the defence as a confessed match-fixer while Riley’s evidence was about alleged confessions from Cairns on a drunken night out. McCullum doesn’t understand why his evidence alone couldn’t be enough to bring about a conviction.

“I simply don’t understand why the jury cannot disregard Lou and Elly’s evidence if they don’t believe it, and still reach a verdict based on whether they believe me or Cairns,” McCullum wrote.

He says he has wondered often since if he made the right decision to put himself through the stress of testifying - something he didn’t have to do.

“I was prepared to stand up, even under pressure and under fire from various quarters, and do what I thought was morally right at the time,” he wrote.