He will be replaced in the squad by his 32-year-old England back-row colleague James Haskell, who is set to link up with the 41-man playing party next Sunday.

Flanker Haskell, who has won 75 caps, is set to be involved for Wasps in the Aviva Premiership final against Exeter at Twickenham 24 hours earlier.

Vunipola was likely to have been an integral part of the Lions' Test series victory bid.

He delivered a magnificent display when Saracens retained the European Champions Cup by beating French heavyweights Clermont Auvergne eight days ago, and also shone in defeat on Saturday as Exeter knocked Vunipola and company out of Aviva Premiership title contention.

The 24-year-old was viewed as a major player in Lions head coach Warren Gatland's squad, although Gatland still has the world-class ability of Wales number eight Taulupe Faletau at his disposal.

But the Lions must now push on without Vunipola after they assembled in Ireland on Sunday night for the second of two pre-tour training camps.

Vunipola, 24, is the second player to pull out following Gatland's squad announcement last month.

Leicester and England scrum-half Ben Youngs withdrew on compassionate grounds after learning that his sister-in-law was terminally ill.

Gloucester scrum-half Greig Laidlaw was called up to replace Youngs, and now Haskell will join up with Gatland's Lions ahead of their departure to Auckland on May 29.

Gatland said: "It is really disappointing for Billy and we know how much he was looking forward to joining the squad.

"He has been carrying an injury and feels he wouldn't be able to contribute fully to the tour and needs further medical treatment.

"We really appreciate his honesty in making this decision and wish him luck with his recovery.

"We have called up James to the squad. We wish him all the best next weekend and look forward to welcoming him into camp before we depart."

The Lions said that 30 players convened at their training base near Dublin on Sunday night.

Numbers will have been boosted following play-off defeats for the likes of Saracens, Leinster and Ospreys, giving Gatland three-quarters of his squad as preparations continue for the tour opener against a New Zealand Provincial Barbarians XV in Whangarei on June 3.

The Lions say that "routine medical screening" will be conducted over the next 24 hours.

Wales hooker Ken Owens missed the Scarlets' Guinness PRO12 semi-final victory over Leinster two days ago because of an ankle injury, while Leinster flanker Sean O'Brien (calf muscle tightness) and Ospreys scrum-half Rhys Webb (groin) were also absentees from play-off action.

The Lions face 10 games in New Zealand, culminating with a three-Test series against the world champion All Blacks that begins in Auckland on June 24.

Reacting to news of Vunipola's injury, former Lions and Ireland captain Brian O'Driscoll tweeted: "Absolutely massive blow to @lionsofficial losing Billy Vunipola. Not sure if there could have been a bigger loss of personnel."

And another ex-Lion - former England centre Will Greenwood - tweeted: "Forget Pragmatism - hammer blow @bvunipola out of @lionsofficial - truly world class player... gutted not to see him up against @AllBlacks."