Ashton will link up with England as their new attack coach, joining supremo Andy Robinson, forwards coach John Wells and defence specialist Mike Ford.
Ashton previously worked with England during Sir Clive Woodward's regime, but he left the Rugby Football Union's national academy six months ago to rejoin Bath as head coach.
However, the lure of international rugby has proved irresistible for one of British rugby's most highly-rated coaches, and he could have a major impact on England as they build towards next year's World Cup defence.
Ashton will link up with the team for the Gartmore Challenge against the Barbarians at Twickenham on Sunday and will fly out to Australia on Monday.
He said: "I've thoroughly enjoyed my time at Bath Rugby and I would like to thank everyone at the club for their support over the last six months.
"The opportunity to return to the senior England coaching set-up was too good to turn down and I'm looking forward to working with Andy Robinson, John Wells, Mike Ford and the squad for the game against the Barbarians and the summer tour to Australia."
Head coach Robinson said: "It's great to have Brian back in the England senior coaching team. He brings a wealth of experience with him and he has proved again at Bath in recent months what an outstanding coach he is."
RFU chief executive Francis Baron said: "Bath have been very co-operative throughout the whole process.
"We have agreed a compensation package with them and I would like to thank Bob Calleja and Bath Rugby for their support, not only during the process of recruiting Brian but also for the England team in spite of some unfair and inaccurate reporting of our discussions in a national newspaper."
Ashton's departure leaves Bath with a considerable void to fill after he steered the club into this season's European Cup semi-finals, when they were beaten 18-9 by French champions Biarritz in San Sebastian.
It was Bath's best European performance since they won the European Cup in 1998, although their Guinness Premiership campaign produced a disappointing ninth-placed finish.
"We are obviously sorry to be losing Brian after such a short space of time." said Calleja.
"He clearly made an impact in that period, and we are now actively seeking a suitable replacement.
"Bath Rugby would like to wish Brian all the best in his new position, and we know that he is well qualified to meet the challenges it will offer him."