''I am hugely honoured to be thought of and very excited to get out there and join up with the squad,'' said Twelvetrees.
''It is a boyhood dream. The other side of my family is Australian and it is something dear to my heart. This will be my first time in Australia. I have always wanted to go out there because part of my family is from there and it does make it special.'
''Like any team you get jumped into, if you don't make your presence known straight away there is no point being there. You get called up for a reason and you have to impose yourself and do the best you can.
''To meet up with players like Brian O'Driscoll and superstars in the game of rugby, I have always looked forward to playing with people like that. To get the chance to train with the best in the northern hemisphere is awesome.''
The 24-year-old won his fifth cap for England in their 32-3 win over Argentina in Salta last weekend, scoring his side's second try, and becomes the second recruit from the England camp following in the footsteps of Corbisiero. He made his England debut against Scotland at the start of this year's RBS 6 Nations.
"The nature of a Lions tour means that there will inevitably be knocks which take time to recover from," said Lions head coach Warren Gatland
"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,"
The Lions currently have Tommy Bowe nursing a broken hand and are hoping to see Ireland full back Rob Kearney make his debut on tour in this weekend's fifth game of 10 in Sydney. With three games to be played in the space of eight days, Twelvetrees will add vital cover should there be any further injury problems or niggles.
England Head Coach Stuart Lancaster said: "All the England players and management 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."