Jones failed to last more than 24 hours with the Lions medical staff, who have requested he undergo further examination by a neurosurgeon.

He suffered what the Lions termed "two significant knocks" to the head while playing for Wales on their recent North America tour.

Jones had been summoned to South Africa as replacement for Ireland flanker Stephen Ferris, who suffered a tour-ending knee injury earlier this week.

The Lions, currently, will not send for a deputy to Jones, who will now fly back to Wales without playing any part on the tour. He only arrived in Cape Town on Thursday morning.

Jones underwent the same routine medical testing as all replacement players joining the tour, and it led to the squad's medical team requesting a further examination by an independent neurosurgeon.

Jones said: "I am very disappointed that on medical grounds I have been told that I am not currently fit enough to play an active role on the tour.

"I was looking forward to getting involved with the rest of the lads, but I guess it wasn't meant to be. It has been an up and down year for me in many ways.

"I feel okay at present, but obviously I have to listen to the neurosurgeon's advice in terms of my health and to ensure I make a full recovery."

On a neurosurgeon's advice, it has been determined it was not safe for Jones to undertake any physical contact for a mandatory three-week period, in line with International Rugby Board regulations on concussion.