Talented players left out, admits Gatland
been unable to force his way into the squad, while Sinckler, Jonathan Davies, Sam Underhill and Josh Navidi will also be left at home.
Sexton finished the Six Nations strongly, but has suffered ongoing concussion problems since.
Sinckler made his name on the 2017 tour to New Zealand and was viewed as odds-on to be in Test contention once more following a strong Six Nations, yet he is missing.
Davies was expected to travel to South Africa, especially after Wales team-mate George North was ruled out by a knee injury, but in a surprise move Aki and Harris are preferred.
Another of Wayne Pivac’s Six Nations champions in Navidi has failed to win a spot in the ferociously competitive back row positions, but Jack Conan of Ireland and Scotland’s Hamish Watson are present.
Underhill missed the Six Nations because of a hip injury but his power in defence was thought to be enough to get a spot, while Billy Vunipola has paid the price for his poor form.
Farrell, Dan Biggar and Finn Russell will compete for the fly-half jersey, but Danny Care has failed to make the cut.
“We believe we’ve picked a squad capable of winning a Test series in South Africa,” said Gatland.
“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 tight calls to make.
“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.”
Gatland is looking forward to taking on South Africa coach Rassie Erasmus, saying: “They’ve got a great coach and what they achieved in the World Cup was outstanding.
“They’re going to be incredibly tough. We know how physical they can be and we saw that in the World Cup final, but we’ve got to be able to roll up our sleeves and hopefully match that physicality, but be really positive in the way we want to play.
“We’ve got some exciting players. And hopefully get that balance right between the tough work but also playing some great rugby.”
Gatland said on the appointment of Jones as captain: “I did panic for a few hours before he rang me back. It was a great conversation.
“There was a bit of apprehension realising the responsibility but he’s done a great job for Wales the last few years. It’s a great honour to be tour captain, but you’ve got to play well as well.”
British & Irish Lions squad – Captain: Alun Wyn Jones (Wal). Forwards: T Beirne (Ire), J Conan (Ire), L Cowan-Dickie (Eng), T Curry (Eng), Z Fagerson (Sco), T Faletau (Wal), T Furlong (Ire), J George (Eng), I Henderson (Ire), J Hill (Eng), M Itoje (Eng), W Jones (Wal), C Lawes (Eng), K Owens (Wal), A Porter (Ire), S Simmonds (Eng), R Sutherland (Sco), J Tipuric (Wal), M Vunipola (Eng), H Watson (Sco). Backs: J Adams (Wal), B Aki (Ire), D Biggar (Wal), E Daly (Eng), G Davies (Wal), O Farrell (Eng), C Harris (Sco), R Henshaw (Ire), S Hogg (Sco), C Murray (Ire), Price (Sco), L Rees-Zammit (Wal), F Russell (Sco), D van der Merwe (Sco), A Watson (Eng), L Williams (Wal).