Reddin has been made redundant from his post as Rugby Football Union head of fitness and nutrition with immediate effect.
He follows head coach Andy Robinson out of Twickenham, completing a dismal year when England also saw coaching support staff Phil Larder (defence), Joe Lydon (attack) and Dave Alred (kicking) all depart the RFU.
Reddin said: "Although the RFU have explained the rationale for their decision to make my role redundant, I am disappointed to be leaving.
"I do feel proud to have been associated with the most successful period in English rugby history, and my hope in leaving is that positive changes are made to recreate that."
Reddin played a key role in England's 2003 World Cup success, but his work with the senior England squad will now be taken up by RFU national fitness coach Calvin Morris.
News of Reddin's exit came as RFU elite rugby director Rob Andrew presented a report to Club England colleagues about the way forward for England following a catastrophic year that produced eight losses in 11 Tests.
The elevation of current attack coach Brian Ashton to caretaker England boss is one possible outcome of Andrew's recommendations to Club England, which will require formal management board ratification on December 20.
Ashton could take on the role for this season's RBS 6 Nations Championship and even the World Cup defence, which begins in France next September.
Andrew is also likely to push for the appointment of an England manager - current Harlequins rugby director Dean Richards would be among the leading contenders - although overseas candidates such as ex-Wasps coach Warren Gatland, former Springboks supremo Nick Mallett an Australia's 2003 World Cup boss Eddie Jones all have their admirers.
Placing Ashton in charge is the probable short-term outcome, with John Wells continuing as forwards coach and Mike Ford overseeing England's defence. Any permanent managerial arrival might not even be this season.
In the meantime, Professional Rugby Players' Association chief executive Hopley has welcomed plans by management board chairman Martyn Thomas to formally co-opt Andrew on to the board.
Hopley said: "This is a significant step forward by the RFU management board, and is fitting given that this is the body that dictates policy regarding elite professional rugby in England.
"The PRA now looks forward to getting representation on the management board so that it is truly representative of all the stakeholders in the game."