The head of world cricket's Anti-Corruption Unit has slammed suggestions it conspired with New Zealand cricket authorities to protect Black Caps captain Brendon McCullum.

Sir Ronnie Flanagan emphatically denied an agreement was made not to follow up on McCullum's alleged non-reporting of an improper approach from former New Zealand all-rounder Chris Cairns in 2008 regarding spot-fixing.

Flanagan described suggestions McCullum was let off the hook and allowed to continue playing international cricket as "absolute, preposterous nonsense" and was just as staunch in his defence of the 34-year-old's integrity and character.

"First of all, I can say, absolutely categorically, there were no deals done with Brendon McCullum," Flanagan told Newstalk ZB. "His reputation is not sullied in my mind and I made that very clear to Brendon.

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"I was actually asked about the possibility of some gain, in respect of not pursuing Brendon McCullum, so that he would be left free to play in the then 2011 World Cup. This is absolute nonsense and no such suggestion of any such deal whatsoever, or any sort of conspiring between Anti-Corruption and between the cricket authorities in New Zealand ... absolute, preposterous nonsense."

Flanagan regretted that McCullum's reputation has been tarnished and said criticism over the fact he took three years before admitting the approach to anti-corruption authorities was unfair.

Under the ICC and ACU regulations at the time, players could report improper approaches to their team captain or team manager and were not obliged to involve the anti-corruption body. The fact McCullum ultimately alerted authorities further cleared him of any impropriety.

"I've picked up a perception in New Zealand, just very recently, when there was news of a person from Hong Kong having been charged and suspended in relation to non-reporting of an improper approach, and some comparisons being made between that and Brendon McCullum," he explained. "There is a world of difference in someone volunteering to come forward to us to tell us something that we otherwise had absolutely no knowledge of.

"There's a world of difference between that - however late that report might be - and us actually finding something out through our investigation and questioning, finding that someone has deliberately withheld something from us. Those are two very different sets of circumstances.

"In fact, the regulations in place at that time were different and actually afforded a player a range of people to whom that player could report. And that included the player's captain or the player's team manager.

"Nowadays they must report it to the Anti-Corruption Unit. It's much clearer and I think it's much more appropriate."

Flanagan accepted the scrutiny McCullum came under and the damage to his reputation once news of his evidence to the ACU was leaked, left international players wary of raising the alarm about any connection to improper advances. The onus remains upon the ACU and ICC to ensure future investigations are better handled.

"I have been speaking to, for example, the New Zealand squad quite recently and issues like this came up. But as I remind the players, they are absolutely obliged to keep this wonderful game of theirs clean and they are... obliged to come forward and assist us in all our investigations.

"It imposes a responsibility upon us to make sure those investigations are carried out to the highest professional standard and in the past we have not always been as good in those areas as we should.

"We operate in a different and in a much more professional manner now than in years gone by and I want to continuously reassure players that that is the case and that of course they must continue to come forward and assist us in keeping this wonderful game clean."