Twelvetrees, who can also play stand-off and starred in England’s 32-3 victory over Argentina last weekend, was due to leave Buenos Aires for Sydney last night.
The Lions have concerns over the fitness of both their stand-offs just ten days before the first Test against Australia, with Owen Farrell suffering from a dead leg and Jonathan Sexton nursing a hamstring problem.
Manu Tuilagi suffered a shoulder stinger in last weekend’s 22-12 victory over the Queensland Reds, Brian O’Driscoll tweaked his groin in Tuesday’s 64-0 win against the Combined Country XV and wing George North has a hamstring problem.
The Lions play the New South Wales Waratahs on Saturday and the ACT Brumbies next Tuesday before the first Test against Australia.
Gatland said: “The nature of a Lions tour means that there will inevitably be knocks which take time to recover from.
“The whole squad have worked hard but a lot of the backs have doubled up over the last two games, either starting or from the bench.
“Billy played well in the first Test for England and he provides that versatility to cover a variety of positions.”
Twelvetrees is the fourth player to be called up by the Lions and the second from England’s tour of Argentina. Alex Corbisiero, the new Northampton loosehead prop, flew from Salta to Australia last week and he has since been joined by the Scotland prop Ryan Grant and Ireland wing Simon Zebo.
Twelvetrees had been due to start England’s second Test against Argentina on Saturday, having been one of the most influential performers last weekend.
Playing at inside centre, Twelvetrees was England’s leading ball-carrier, breaking the gain-line 12 times and he scored the second try of his Test career.
He has played the majority of his rugby at inside centre but he can operate at No 10 and he will offer a ball-playing and goal-kicking option to the Lions.
England coach Stuart Lancaster said: “All the England players and management are delighted for Billy to get this great opportunity.
“He’s played well for England this year, underlined that form with his display against Argentina at the weekend and goes with our full support and blessing.”
Meanwhile, Michael Cheika has ruled out any repeat of a violence-scarred 2001 clash between New South Wales and the British and Irish Lions in Sydney this weekend.
The game 12 years ago is still remembered for the Waratahs’ former Saracens and Gloucester stand-off Duncan McRae punching Lions back Ronan O’Gara 11 times while the Irishman lay prone on the floor.
O’Gara subsequently had eight stitches put in a facial wound, while McRae received a seven-week ban following an incident that left a bitter taste and commanded global television replays.
“I think those days have gone,” current Waratahs head coach Cheika said yesterday. “Even back then, it was over the top. But physicality can occur in lots of ways – rucks, scrums and the contacts have got to be big from our lads. We have to be big. We can’t hope just to contain them, and we can’t hold anything back.
“If we have that mentality over 80 minutes, we might put a bit of a dent in their tank.
“Anything in red that moves we’ll have a go at it.”
Former Leinster and Stade Francais boss Cheika is without ten players this weekend who are away with Australia currently preparing for next week’s first Test against the Lions.
He has also has a number of players unavailable through injury, but he added: “We certainly won’t be going out there to look for a glorious defeat.
“If there is a one per cent chance we can win the game we’ll take that one per cent and do what we can with it.”