Burns, 22, replaces Jordan Turner-Hall, who will return to Harlequins for treatment on an inflamed knee, while centre Twelvetrees, 23, comes in as temporary cover for Jonathan Joseph, whose ankle injury is being closely monitored.
Meanwhile Launchbury, 21, steps in as cover for Tom Palmer, who is a doubt with a calf strain.
The Cherry & Whites pair are both uncapped by England, but have both put themselves in contention for Six Nations consideration with starring performances for Nigel Davies’ side so far this season.
Livewire fly-half Burns, the leading Aviva Premiership points-scorer with 124, landed a flawless seven from seven attempts at goal in Saturday’s 28-25 defeat to Quins at the Stoop, while Twelvetrees scored a try to earn Gloucester their bonus point.
And Lancaster believes the double call up is a good opportunity for the pair to impress, and insists preparations are well underway ahead of Saturday’s clash against Fiji at Twickenham.
“Freddie has been playing well in the Aviva Premiership and, with Jordan needing to stay at Harlequins, we are keen to have a look at him,” he said.
“JJ is continuing his recovery from an ankle injury and this is a good chance to get Billy in for training. George Robson and David Paice again come in as injury replacements for Courtney Lawes and Dylan Hartley.
“We had a productive week at St George’s Park and got a lot of work done. Now we are back at Pennyhill we will sharpen our focus towards Fiji on Saturday.
“Both will be assessed and treated in camp and it is important that they stay with the squad as much as possible.”
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BIOGRAPHY: Marcus Rashford