Fly-half Burns replaces Harlequins centre Jordan Turner-Hall, who is suffering from an inflamed knee.
While Twelvetrees also links up with the England at Pennyhill Park as temporary injury cover for Jonathan Joseph, the London Irish centre who is carrying an ankle injury.
A total of 36 players were scheduled to convene, including Dylan Hartley and Courtney Lawes who will have their respective knee injuries assessed by the England medics.
England play Fiji at Twickenham on Saturday and then face Australia, South Africa and New Zealand in a challenging autumn campaign.
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The Barbarians are one of the top invitational rugby sides with a long history and classic rugby jersey.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Lions get ready for their matches with this green training jersey. It's what the players wear. Get in the Gear!
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The Lions are ready to get their Australia tour underway. They arrive in Perth on Monday.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.