Vice-captain Berrick Barnes has been ruled out of the Wallabies' grand slam tour of the UK and Ireland with a serious ankle ligament tear.

The playmaker is now expected to undergo "several months" of rehabilitation.

In a devastating blow to Barnes and Australia's prospects of completing a rare sweep of England, Ireland, Scotland and Wales, the Wallabies learned late on Friday night that he had a nasty syndesmosis injury and would have to be sent home.

"Obviously you never enjoy injuries, particularly from a player's perspective because they work very hard," Wallabies coach Robbie Deans said.

"So to see them suffer misfortune and miss out on the opportunity to play isn't easy and isn't much fun for anyone in the group."

Earlier on Friday, Matt Toomua was drafted into the squad as cover for Barnes, but the Wallabies had been hoping the Brumbies teenager would only be required for Tuesday night's match against English club side Gloucester.

An MRI scan on Barnes' left ankle - after he rolled it on Thursday and was ruled out of Saturday's Bledisloe Cup Test against the All Blacks in Tokyo - had proved inconclusive.

But further medical advice confirmed Barnes's worst fears - that he'd be flown home early from the spring tour for the second year running.

Barnes broke down with a knee injury less than 20 minutes into last year's tour.

Toomua, who was to arrive in London on Saturday, is the second Brumbies player added to the touring party this week after centre Tyrone Smith - younger brother of Test stalwart George - arrived on Thursday to replace NSW Waratahs youngster Rob Horne, who tore his hamstring at training.

"This is a great development opportunity for Matt, as it is for his Brumbies team-mate Tyrone Smith, who joined us earlier in the week," Wallabies coach Robbie Deans said.

"We had always planned to restrict the involvement of the Test playing staff in this match as much as we could, to allow those individuals to recover from the match in Tokyo on the Saturday.

"By having 19 players available to us who didn't start in Tokyo, we can ensure those who started against the All Blacks can recover properly while providing a valuable development opportunity to a wider group of players at Gloucester.

"We can then settle on a combination for the Test against England the following weekend at Twickenham."

Three players who start in Saturday's Test in Tokyo will be required for the match-night 22 at Gloucester, but Deans said they would only be named on the bench.

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