He started last Saturday’s first Test defeat against South Africa in Durban, but was replaced by Ireland full-back Rob Kearney just before half-time.

Kearney retains the number 15 shirt for the second Test in Pretoria this weekend.

Ospreys star Byrne was tipped to be a star of the tour, given his outstanding form in Wales colours last season, but injury has now put paid to him playing any further part in the Test series.

He made four starts on the 10-match trip - against South Africa, the Royal XV, Cheetahs and Sharks - scoring two tries.

The Lions management revealed Byrne suffered his injury during training in Cape Town today.

At this stage of the tour, there are no plans to summon a replacement.

Byrne said: “I am very disappointed that I got injured in training today and that my tour is over.

“I was recovering from a foot injury and was taking part in training when I hurt my thumb in a freak accident.

“To be ruled out of the tour at this stage with just two matches left is devastating.

“Returning home immediately is the right thing to do as it means I can get the right medical treatment straightaway. I wish the boys all the best for the rest of the tour.”

Byrne is the latest player to make an early exit from South Africa, following his Wales colleagues Leigh Halfpenny and Ryan Jones, Ireland flanker Stephen Ferris and Scotland prop Euan Murray.

Injury also hit Tomas O’Leary, Jerry Flannery and Tom Shanklin - all original tour selections who did not make it to South Africa - while a 12-week suspension sidelined Munster flanker Alan Quinlan.