David Campese has questioned whether Warren Gatland has a Plan B and if Wales are capable of changing tactics should they be forced to chase a game at the World Cup.

Rugby great 'Campo', whose brilliance was a key in Australia conquering the world in 1991, is adamant the European champions’ success story is based largely on defensive excellence.

"The Welsh are a team we have got to worry about. We know how Wales play – their defence is very good," said one of rugby’s greatest entertainers of all-time.

"They’re not about attack and the key to beating them is putting them under pressure.

"If you can break up their defence a bit and get some points on the board it will be interesting to see how they react.

"Would they be able to change tactics and switch to an attacking game if they have to chase a game?"

Wales were crowned Six Nations Grand Slam kings during a record winning run which has seen them triumph in 15 of their last six fixtures to become the No.1 ranked team in the world for the first time in history. You can see how those rankings could change today, here.

That streak saw them beat Australia for the first time since 2008 as they prepare for a Pool D showdown with the Wallabies next month at the World Cup in Japan.

Wales celebrate beating Australia at the Principality Stadium in November last year

Former backs magician Campese says Wales lifting this year’s Six Nations ahead of England and Ireland has earned them new respect Down Under.

But he pointed out the World Cup is a series of one-off matches, saying: "You can’t worry about history, it’s the here and now that counts.

"There are some good teams in it and I think it’s an open tournament which could come down to a refereeing decision or an injury," Campese continued.

"We saw the effect the sending off of Welsh skipper Sam Warburton against France in the 2011 semi-finals had on that match (Wales lost by a point) and Scott Barrett being red-carded for the All Blacks against Australia a couple of weeks ago with the Wallabies winning by a record score."

Two-time winners Australia, who were beaten in the 2015 final by New Zealand at Twickenham, face Wales, Fiji, Georgia and Uruguay in Pool D.

Campese echoed the words of another Wallaby great, outside-half Michael Lynagh, by stressing the Aussies are more "worried" about facing Fiji in their opening fixture of the tournament than what comes afterwards.

"Pool D is a tough one and I think we are lucky we have got Fiji first up," said Campese.

"The smaller nations don’t have the same level of preparation matches as the top eight countries so it can take them time to get going.

Australia captain Nick Farr-Jones (left) and David Campese lift the World Cup after Australia's 12-6 victory over England in the 1991 final

"By the time Fiji face Wales (it’s their final match in the group) you’d expect them to be in the swing of it.

"If Australia or Wales don’t turn up on the day they’re in trouble if Fiji do.

"The biggest problem Fiji have had in the past is they’ve not been good to go for the full 80 minutes. They would last 50 or 60 and blow up.

"That comes from not playing enough games against the big boys to get them up to speed. The World Cup is loaded against the smaller nations in that respect.

"If they had the same preparation as the major nations and all their top players were available to play for them rather than having been pinched by other countries it would be better for world rugby."

Campese listed his favourites in order as New Zealand, who are bidding for an historic three World Cups in a row, resurgent South Africa, England, Wales and Ireland.

"It could come down to strength in depth and who has the strongest bench because that’s often the biggest difference nowadays," he said.

"I'm looking forward to it. You don't know what impact Argentina and France, who always have one big game in them, are going to have against England in their group while hosts Japan will be targeting Scotland."