Ashton has bagged three tries in three Tests against the Wallabies and is set to line-up on the left wing, with Charlie Sharples, who scored a brace against Fiji, on the right.
"It is tough on Ugo, but Chris has been excellent in training and Charlie is rewarded for his performance at the weekend,' said Lancaster.
Ashton's call up is the only change to England's squad, with Lancaster set to name his side tomorrow.
Alex Corbisiero and Jonathan Joseph have both returned to action following injuries but Lancaster was reluctant to rush them back to international duty prematurely.
Manu Tuilagi has not trained this week due to the ankle injury he picked up against Fiji but England are confident he will be passed fit.
"We had to make some tough decisions within the 23 and we are in a good position to be able to select from a largely fit squad," added Lancaster.
"Alex Corbisiero has played once in five months so it is important that he goes back and has more game time for London Irish.
"Jonathan Joseph has come through training but Brad Barritt and Manu did well in the centres and, similar to Alex, it would be good for him to get some game time."