Ryan Jones suffered a head injury early on against United States
Wales captain Ryan Jones has been declared unfit for Lions duty and began his journey home on Friday, a day after joining the Lions in South Africa.
The 28-year-old's plight was caused by the head injury suffered in the first minute of Wales' 48-15 win over United States in Chicago on Saturday.
Jones joined the Lions on Thursday as Irishman Stephen Ferris' replacement.
But Lions head coach Ian McGeechan says no further replacement will be called up at this stage.
On the day Jones joined the tourists, Wales boss and Lions forwards coach Warren Gatland said Jones might not have had time to stake a Test place claim during the remainder of the tour.
But even that slim opportunity was then denied to Jones, whose tour was ended following a medical examination in South Africa.
He joins Tomas O'Leary, Tom Shanklin, Jerry Flannery, Leigh Halfpenny and Ferris in having his Lions dreams shattered by injury.
It is a further blow to Jones, who was controversially omitted from the initial Lions tour party.
A statement on the Lions website read: "Jones underwent the same routine medical testing as all replacement players joining the tour and it led to the team's medical staff requesting a further examination by an independent neurosurgeon.
"This was due to a concern over the player's recent medical history that included two significant knocks to the head while playing for his country on their recent tour to North America.
"On the advice of the neurosurgeon, it was determined that it was not safe for Jones to undertake any physical contact for a mandatory three weeks."
A Welsh Rugby Union statement read: "Ryan Jones was initially examined by the Wales squad medical team in the USA and was passed fit to travel home to the UK.
"A full medical examination of the player was due to take place in the UK late this week but the routine tests were then scheduled for South Africa when he flew out on Tuesday."
A WRU spokesman added: "We fully understand the reasons this step has been taken, but we understand how desperately sad it is for Ryan who was looking forward to the honour of representing the Lions.
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
"All players know it is important for medical advice to be followed and now he will be able to rest before taking a full part in pre-season training.
"We look forward to seeing him fit and ready for the start of next season."
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."
Jones was a member of Wales' 2005 Grand Slam back-row and was one of the few Lions tourists to New Zealand of that year to shine during a 3-0 Test whitewash.
He went on to lead Wales to their 2008 Grand Slam success, but faced criticism for his Wales and Ospreys display during the 2008-09 domestic season.