England's RBS 6 Nations campaign kicks off against Scotland at Twickenham on February 2 with the team heading up to Leeds at the end of next week to begin their preparations.
The inclusion of Burns and Twelvetrees comes as a reward for Gloucester's fine start to the season while Launchbury and Haskell were both involved in England's triumph over the All Blacks last month.
Strettle's inclusion comes at the expense of Charlie Sharples who has to settle for a place in the Saxons squad while Youngs started every game this autumn in the absence of the injured Dylan Hartley and comes in for the injured Rob Webber.
"The fact that we have spent significant time deliberating over quite a few players shows how tough the competition is to get into this 65 and the strength in depth that is emerging," said Lancaster.
"We have made some changes to both squads and we feel that reflects the players' form during the first half of the season.
"There will undoubtedly be movement between the two squads as we go through the RBS 6 Nations and that was evident in the QBE Internationals, with players in form pushing through such as Freddie Burns, Joe Launchbury, Tom Youngs and Mako Vunipola.
"This is a very important year for England, with the Six Nations and the Saxons games to look forward to so there are some great opportunities for the players to demonstrate their club form at international level.
"The summer tour to Argentina will also be a great chance for those players pushing for a place, especially with some of our Senior squad hopefully going with the Lions, and chances will undoubtedly arise as we hope to play two midweek games as well as the two Tests."
Meanwhile in the Saxons there are a number of new faces including Wasps duo Elliot Daly and Billy Vunipola, with the No.8 set to train with the first team while Tom Croft recovers from injury.
Former league stars Joel Tomkins and Kyle Eastmond are also included for the first time ahead of the games against the Irish Wolfhounds and Scotland A.
England senior squad:
Calum Clark (Northampton Saints)
Alex Corbisiero (London Irish)
Dan Cole (Leicester Tigers)
Tom Croft (Leicester Tigers)
Dylan Hartley (Northampton Saints)
James Haskell (London Wasps)
Tom Johnson (Exeter Chiefs)
Joe Launchbury (London Wasps)
Courtney Lawes (Northampton Saints)
Joe Marler (Harlequins)
Ben Morgan (Gloucester Rugby)
Geoff Parling (Leicester Tigers)
Chris Robshaw (Harlequins)
Mako Vunipola (Saracens)
Thomas Waldrom (Leicester Tigers)
David Wilson (Bath Rugby)
Tom Wood (Northampton Saints)
Tom Youngs (Leicester Tigers)
Chris Ashton (Saracens)
Brad Barritt (Saracens)
Mike Brown (Harlequins)
Freddie Burns (Gloucester Rugby)
Danny Care (Harlequins)
Lee Dickson (Northampton Saints)
Owen Farrell (Saracens)
Toby Flood (Leicester Tigers)
Ben Foden (Northampton Saints)
Alex Goode (Saracens)
Jonathan Joseph (London Irish)
David Strettle (Saracens)
Manusamoa Tuilagi (Leicester Tigers)
Billy Twelvetrees (Gloucester Rugby)
Ben Youngs (Leicester Tigers)