The 45-year-old had vowed to carry on in his role in the wake of Saturday's 5-0 defeat at Blackpool on Saturday afternoon, but it now appears that both parties have agreed to part ways.
"I'd like to place on record my thanks to Keith for his 12 years of service at the club. He is an outstanding coach and we wish him all the very best for the future," chairman Colin Sexstone told the club's official website.
The club have also confirmed that they hope to have appointed a replacement in time for the Robins' next match at home to Peterborough United on Saturday, October 15.
Millen has become the second Championship manager to leave his job in less than 24 hours following the resignation of Steve McClaren at Nottingham Forest yesterday.