As Boris Johnson prepares to become the UK's next prime minister, his former school rugby coach reminisces on a clever kid who "stood out".

Ian McKinnon, now a Wellington regional councillor, was Deputy Headmaster at Eton College in Britain in 1978 - when Boris Johnson was just 14 years old.

While McKinnon didn't teach the young Johnson, he did coach his rugby team.

He remembers him as one of the 70 King's Scholars at the school at the time, called Collegers.

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"Collegers aren't noted for their rugby playing but Boris was a bit of an exception because he played rugby and was a robust forward, and certainly stood out for the same reasons he continues to stand out today," McKinnon told Newstalk ZB's Heather du Plessis-Allan.

"He stood out at Eton because as a Colleger they have to wear an academic gown, which is pretty rare for schoolboys to wear, and I can remember him arriving late to most things that he was expected to be at – gown flying in one direction and a flock of blonde hair flying in the other.

"I am not certain he has changed that much in the intervening years," he laughed.

While tardy, McKinnon said Johnson has always been very clever.

"He is a classicist, he's a Latin and Greek scholar and he was clever at Eton.

"I think that his track record shows that what everyone thinks of his politics – he is a pretty able person academically."

McKinnon said Johnson's impending political standing was pretty well summed up in his book The Churchill Factor.

"Essentially if you read his book, I think you will have a fair understanding of Boris Johnson himself.

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"He admires Churchill greatly. He was a bit of a rebel, a bit of an independent, and I think Johnson's a little bit of the same."

McKinnon said it's this rebellious nature that could mean Johnson sinks or swims in the new position.

"The first thing that will happen is some of the media will realise for all of the dumping they were doing on Theresa May, she was in a pretty difficult situation - and I suspect Boris Johnson will find himself in an equally difficult situation.

"With his personality, rebellious nature and independent thinking - is he able to achieve what Theresa May couldn't?" he quipped.

"I think James Blitz, reporter for the Financial Times, summed it up very well when he said 'the difficulty with Boris Johnson is he has basically laid down a binary choice – it's either the deal he wants or it's no deal and he's out'.

"In other words he is not going to pussyfoot around too much - That's not quite the way you've got to negotiate to succeed, possibly," McKinnon said.

Johnson is the overwhelming favourite to be crowned Conservative Party leader, with an announcement due around 10.45pm (11.45am BST).

He is likely to win by a substantial margin over his rival, Jeremy Hunt, and will then make a victory speech.

Theresa May, who is standing down after a revolt by Conservative MPs over her Brexit policy, will officially tender her resignation to the Queen on Wednesday.

Johnson is set to take office at a time of political crisis over Brexit, but he's promised to seek a new deal with the EU or leave without an agreement by the end of October.