Centre Mike Tindall is set to miss the remainder of England's RBS 6 Nations campaign after suffering bruising to his liver that meant he was admitted to Hammersmith Hospital in London, where he remains under observation.
Wing David Strettle (foot), together with flankers Lewis Moody (Achilles) and Tom Rees (knee ligament) are also out of the Italy game and have returned to their clubs.
Ashton has now added uncapped Newcastle scrum-half Lee Dickson, London Irish back Shane Geraghty and Sale Sharks flanker Magnus Lund to the squad.
Dickson, 22, was born in Germany and represented Scotland at under-19 level before switching his international allegiance to England and being part of their Under-21 World Cup campaign in 2005.
He helped England Saxons record an impressive 31-13 victory over Ireland A last Friday night, a game that saw Geraghty named man-of-the-match.
Newcastle rugby director John Fletcher said: "I am delighted for 'Dicko' to receive his first England call-up.
"He is continually working hard to improve his game.
"He played well for the Saxons last week, and this is just another acknowledgement of the quality of young English players coming through at the club right now."
Geraghty, who switched from centre to fly-half when Gloucester's Ryan Lamb went off injured, won two caps as a replacement last season against France and Wales.
Lund also has England Test experience, having collected 10 caps, although he was among the first batch of players omitted by Ashton from World Cup contention last summer.
Ashton is due to announce his line-up for Italy on Wednesday afternoon.
Gloucester wing Lesley Vainikolo, Newcastle centres Jamie Noon and Mathew Tait, Bath hooker Lee Mears and his club colleague, flanker Michael Lipman, are likely to be among the players considered as possible starters.