The move was unexpected because Foster and Partners was a member of the Southwark Land Regeneration consortium, which walked away from the scheme after an acrimonious dispute with Southwark council.
SLR, which was led by developer Godfrey Bradman, had reached preferred bidder stage when the deal broke down in April.
The council said at the time that it was unlikely to retain any of SLR's architects on the scheme.
Foster has been asked by the council to draw up a vision for the area that will in effect create a town centre. This will be a large part of the brief given to developers next year when the council produces tender documents.
A source close to Foster said: "Southwark was unable to proceed with SLR, and that left the project in limbo. The council sees Foster as offering a sense of continuity. What the council wants is help with visioning – it wants a document that can then be taken out to developers."
The source added that Foster has a month to decide whether it wants to work with Southwark council. The dilemma for Foster is that it takes the commission, it may not be able to bid for the project.
The council is seeking legal advice over this.
Foster is understood to want to bring back members of the original architectural team, such as Ken Yeang and HTA, but the council is arguing this is impractical because of the limited scope of the work.
John Abbott, the council's assistant project director for the Elephant & Castle scheme, said: "We are looking to bring someone into the process, but we haven't made a final decision. We need an independent architectural input to flesh out our principles."
A project observer added: "It's a slightly unusual move. But at least Foster knows the issues."
Ben Derbyshire, director at HTA, said: "The Elephant & Castle needs incisive vision and Foster at his best has the capacity and the genius necessary to deliver that."