Johnny Sexton's omission from the British and Irish Lions squad was one of Warren Gatland's "toughest decisions", the coach has admitted.

Ireland legend Sexton, the 2018 World Rugby Player of the Year, was one of the surprise exclusions from Gatland's 37-man selection for the upcoming tour of South Africa.

Sexton misses out despite the fly-half playing a key role on the 2017 trip to New Zealand, with Gatland expressing some concerns over the 35-year-old's "durability" and instead calling up Dan Biggar, Owen Farrell and Finn Russell.

"There's no doubt that that was one of the toughest decisions that we had to make," said Gatland at a news conference on Thursday.

"I've got a huge amount of respect for Johnny and his quality, but it's such a pivotal position for us and it's about sending a message to the 10s that we have selected that we have the confidence and belief in them to do a job.

"When we looked at where we were going in South Africa probably the thing that kept coming back to us was Johnny's durability, being able to put a string of matches together consecutively, and big matches.

"He has been rested on a number of occasions and unfortunately that's possibly what swayed it for us in terms of us not having the confidence that he can get through what's going to be a tough, physical tour."

Billy Vunipola was also a notable absentee from the list, while other big calls saw Wales centre Jonathan Davies, England prop Kyle Sinckler and Ireland second row James Ryan left out.

However, Gatland did find room for Exeter Chiefs number eight Sam Simmonds, who has not played a Test for England since March 2018.

The Lions will face the Springboks in Tests on July 24, July 31 and August 7.

Alun Wyn Jones has been named captain of the British and Irish Lions for their tour of South Africa, leading a 37-man squad.

Warren Gatland announced his selection on Thursday ahead of the trip in July, which follows a warm-up against Japan next month.

Wales great Jones has been chosen for the leadership role having featured in the past nine Lions Tests going back to 2009, the longest run in the professional era.

Jones has already skippered the side once previously in the absence of the injured Sam Warburton in Australia eight years ago, clinching a first series win since 1997.

Elsewhere in Gatland's squad, Ireland's former World Rugby Player of the Year Johnny Sexton misses out despite playing a key role on the 2017 trip to New Zealand.

Billy Vunipola was also a notable absentee from the list, while other big calls saw Wales centre Jonathan Davies, England prop Kyle Sinckler and Ireland second row James Ryan excluded.

However, Gatland did find room for Exeter Chiefs number eight Sam Simmonds, who has not played a Test for England since March 2018.

"We believe we've picked a squad capable of winning a Test series in South Africa," Gatland - who opted for 11 Englishmen, 10 Welshmen and eight apiece from Ireland and Scotland - said following the squad announcement.

"Selecting a Lions squad is never easy and, in many ways, this has been the most challenging selection I have been involved in.

"Over the course of the last three weeks the coaches and I have rigorously debated each position.

"We saw some outstanding performances in the recent Six Nations, so competition for places has been tough with some incredibly tight calls to make. 

"However, we are very happy with the squad we have assembled and look forward to meeting up in Jersey in just over a month's time to start our preparation to take on the world champions.

"We've left out some very talented players which gives an indication to the strength of this squad and we know how important that stand-by list will be.

"Being selected for a Lions tour is the greatest honour for a British and Irish player and I congratulate everyone named today."

The Lions will face the Springboks in Tests on July 24, July 31 and August 7.

British and Irish Lions squad in full:

Josh Adams (Cardiff, Wales), Bundee Aki (Connacht, Ireland), Dan Biggar (Northampton Saints, Wales), Elliot Daly (Saracens, England), Gareth Davies (Scarlets, Wales), Owen Farrell (Saracens, England), Chris Harris (Gloucester, Scotland), Robbie Henshaw (Leinster, Ireland), Stuart Hogg (Exeter Chiefs, Scotland), Conor Murray (Munster, Ireland), Ali Price (Glasgow Warriors, Scotland), Louis Rees-Zammit (Gloucester, Wales), Finn Russell (Racing 92, Scotland), Duhan van der Merwe (Edinburgh, Scotland), Anthony Watson (Bath, England), Liam Williams (Scarlets, Wales); Tadhg Beirne (Munster, Ireland), Jack Conan (Leinster, Ireland), Luke Cowan-Dickie (Exeter Chiefs, England), Tom Curry (Sale Sharks, England), Zander Fagerson (Glasgow Warriors, Scotland), Taulupe Faletau (Bath, Wales), Tadhg Furlong (Leinster, Ireland), Jamie George (Saracens, England), Iain Henderson (Ulster, Ireland), Jonny Hill (Exeter Chiefs, England), Maro Itoje (Saracens, England), Alun Wyn Jones (Ospreys, Wales), Wyn Jones (Scarlets, Wales), Courtney Lawes (Northampton Saints, England), Ken Owens (Scarlets, Wales), Andrew Porter (Leinster, Ireland), Sam Simmonds (Exeter Chiefs, England), Rory Sutherland (Edinburgh, Scotland), Justin Tipuric (Ospreys, Wales), Mako Vunipola (Saracens, England), Hamish Watson (Edinburgh, Scotland).

Alun Wyn Jones has been named captain of the British and Irish Lions for their tour of South Africa, leading a 36-man squad.

Warren Gatland announced his selection on Thursday ahead of the trip in July, which follows a warm-up against Japan next month.

Wales great Jones has been chosen as captain having featured in the past nine Lions Tests going back to 2009, the longest run in the professional era.

Jones has already skippered the side once in the absence of the injured Sam Warburton in Australia eight years ago, clinching a first series win since 1997.