A statement from the ECB said: "The selectors have been pleased to learn that the treatment Marcus has been receiving throughout the summer has resulted in very positive progress and that he is expected to be fit to resume playing for England prior to the Ashes tour."
A spokesman from Performance Healthcare, the specialists treating Trescothick, said: "After his return from the tour of India in March, Marcus sought specialist help for his ongoing symptoms. In addition to the deleterious effects of the acquired gastrointestinal infection on his health, it later became evident that he was also suffering from an underlying stress-related illness.
"He has been receiving specialist treatment, which has allowed him to resume his position in the England team. However, we believe that it would be premature for him to tour India in October. Rest is an important part of his treatment and he will need recovery time before the Ashes tour of Australia which begins in November."
Trescothick, a lynchpin of the side, had been in poor form lately and managed just 22 runs in the three ODIs against Pakistan. Despite the positive tone of the ECB statement, this must raise doubts whether Trescothick will be in a state to participate in the Ashes.
To add further weight behind the theory that Trescothick's future is in doubt, Duncan Fletcher has issued a warning about players missing tours. Fletcher, who is due to meet with fellow selectors David Graveney and Geoff Miller said: "I've said all along we've just got to come up with a method of making sure players don't pick [and chose] these tours. I would want my players available for all tours that are played."
But, despite England's seventh ODI defeat of the season, Fletcher insists that if they can play a full-strength team they can still compete. "When we had our full side, close to our Test side, we got to the Champions Trophy final [against West Indies at The Oval in 2004] and we competed very well with [world champions] Australia.
"Since then we haven't had that side. Continuity is important but you can't have continuity if people keep coming in the side because of injuries."
England's selectors meet in Leicester today to pick the Champions Trophy squad, although it will not be named until Sunday. Trescothick's opt-out will cause them yet another headache.