Publications
Partners
Twelvetrees, who scored a try in England's 32-3 win over Argentina in the first test on Saturday, will provide cover for the injury-hit Lions.
Ireland winger Simon Zebo was also called into the Lions squad over the weekend following an injury to winger Tommy Bowe.
Centre Manu Tuilagi and flyhalves Jonny Sexton and Owen Farrell are all carrying minor injuries.
"The nature of a Lions tour means that there will inevitably be knocks which take time to recover from," coach Warren Gatland said.
"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."
England coach Stuart Lancaster said he was delighted for Twelvetrees to get this 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," Lancaster said.
The first of three Lions tests against Australia is on June 22.
England will name their team for the second test in Argentina on Thursday.
