By Simon Austin

Paul is keeping Will Greenwood out of the England side

England centre Henry Paul says Saturday's showdown with South Africa will be the biggest game of his career.

Paul has starred in both rugby's codes, winning 23 caps for New Zealand and the Grand Final with Bradford in league, and four caps for England in union.

But he says Saturday's game is the most important of all as it offers a chance to cement his England place.

"It is totally the biggest game of my career - this is such a massive opportunity," he told BBC Sport.

"You can tell what a big game this is. No-one's smiling or joking around in training.

"Once we're on the training pitch it's serious business. It'll be a massive atmosphere at Twickenham and there's great rivalry between the sides."

Paul, 30, made his England debut against France in March 2002 but has gone on to win only three more caps.

However, he kept World Cup winner Will Greenwood out of the side against Canada last weekend and has retained his place against South Africa.

He will be up against South Africa's big, powerful centres Marius Joubert and De Wet Barry at Twickenham on Saturday.

Paul's defensive capabilities have been questioned in the past, but the affable Kiwi is confident he will be up to the task on Saturday.

South Africa's De Wet Barry (left) is a powerful runner

"Andy [Robinson] and the coaches have faith in me and I have to back that up," he said.

"I'm pretty confident. I have to work hard this week and be a player that those inside and outside me can really trust.

"We seem confident and did well in the game against Canada on Saturday.

"But we obviously had a lot of time and space. It will be interesting now to see what happens with less time and space against two big guys in the centre."

Paul has been working hard with England's renowned defensive coach, Phil Larder, over the last two weeks.

"Learning the calls and the nuances of the players inside and outside me has been the hardest thing defensively," he said.

"There are a lot of things you have to learn all the time.

"At this level the attention to detail is massive. You have to make sure all the boxes are ticked."

Although Paul's switch to union has not always gone entirely to plan, he says he has never regretted his decision to switch codes and join Gloucester in 2001.

"It never once crossed my mind, it was never tough," he said.

I don't think in either code I was ever the complete player - you're always striving to improve Henry Paul

"I came to the sport to win trophies with Gloucester and anything else was a bonus.

"I really enjoy playing for Gloucester now. When I first went there, they were known as a team that just put the ball up the jumper and never really won a trophy.

"In my first year we were there or thereabout for winning trophies.

"Now I'm part of the this England squad as well it's great and I'm really going to try to enjoy it.

"What's happened in the past has hopefully given me the experience to help me through this game against South Africa."

And despite being a professional rugby player for more than 10 years, he is still desperate to develop his game.

"I don't think in either code I was ever the complete player, you're always striving to improve," he said.

England will hope that development continues on Saturday.