Williams played the last of his 87 Tests for Wales against Australia in 2011, scoring his 58th international try with his last touch of the game. His last game on the international stage was for the Barbarians against Wales in 2012 and he is currently in his second season playing in Japan. He was on the Lions tours of New Zealand in 2005 and South Africa in 2009 and has played in four Tests.
Twelvetrees arrived in Sydney from Argentina in time to watch the Lions beat the HSBC Waratahs 47-17 on Saturday, while Wade was pulled out of the England side to face the Pumas in the second Test on Saturday in order to arrive in Australia as soon as possible.
The Saracens centre Barritt was on holiday in the USA and is due in Sydney on Monday. The replacements have been called up because of on-going injury worries surrounding Tommy Bowe (broken hand), George North (tight hamstring) and Manu Tuilagi (shoulder).
In addition, Jamie Roberts limped out of the win over the Waratahs with a right leg injury that is being scanned today to try to discover what is wrong.
The Lions made it five wins from five on Saturday night and are still on course to complete their tour undefeated. But Tuesday night's final run out before the first Test at the weekend is likely to be the toughest game yet against a Brumbies side that dominated the Australian Conference in Super Rugby.
Ulster and Ireland hooker Rory Best will captain the side and Scottish full back Stuart Hogg makes his second appearance on tour in the No 10 jersey following his successful outing against the Combined Country XV.
Rob Kearney gets his first start on tour after coming on as a replacement in Sydney, while scrum half Ben Youngs is the only other player in the starting XV who had game time against the Waratahs.
British & Irish Lions (v Brumbies)
15. Rob Kearney (Leinster / Ireland)
14. Christian Wade (London Wasps / England)
13. Brad Barritt (Saracens / England)
12. Billy Twelvetrees (Gloucester / England)
11. Shane Williams (Mitsubishi Dynaboars / Wales)
10. Stuart Hogg (Glasgow Warriors / Scotland)
9. Ben Youngs (Leicester Tigers / England)
1. Ryan Grant (Glasgow Warriors / Scotland)
2. Rory Best (Ulster / Ireland, captain)
3. Matt Stevens (Saracens / England)
4. Ian Evans (Ospreys / Wales)
5. Richie Gray (Scotland)
6. Sean O'Brien (Leinster / Ireland)
7. Justin Tipuric (Ospreys / Wales)
8. Toby Faletau (Dragons / Wales)
16. Richard Hibbard (Ospreys / Wales)
17. Alex Corbisiero (London Irish / England)
18. Dan Cole (Leicester Tigers / England)
19. Geoff Parling (Leicester Tigers / England)
20. Dan Lydiate (Dragons / Wales)
21. Conor Murray (Munster / Ireland)
22. Owen Farrell (Saracens / England)
23. Simon Zebo (Munster / Ireland)