Developer European Land invited the practice to join the Southwark Land Regeneration consortium only weeks before the 31 March deadline for submissions.
It is understood that Malaysian star architect Ken Yeang is also joining the team, which includes HTA Architects, Benoy and KP Architects.
Yeang, famous for his tower designs, was due to meet HTA this week to discuss his involvement. Southwark Land Regeneration also includes management services firm Enterprise and French construction giant Bouygues.
Foster's role is unclear at the moment. A spokesperson for the practice said the part that it will play on the project has yet to be decided.
Southwark Land Regeneration was one of three consortia shortlisted by Southwark council for the 40 ha scheme last September. The other two are the London & Amsterdam/Countryside consortium, which has Terry Farrell and Michael Hopkins on board, and the St George consortium, which includes Land Security and architect Tibbalds Munro.
Michael Hill, a director of Countryside Properties subsidiary Countryside In Partnership, said his team had a fairly clear view of its masterplan.
It cannot come much bigger or much more exciting than this
Michael Hill, London & Amsterdam/Countryside
He said: "It cannot come much bigger or much more exciting than this. We need to harness the potential of the area as fully as we can." Public consultation on the masterplans will begin in April, when Southwark council will hold an exhibition. A scheme will be picked in summer.
The masterplans will cover urban regeneration, transport and infrastructure, and housing and social issues.
The development of Elephant & Castle, which includes some of the most deprived areas of London, is due to be completed in 2014. The project has been split up in several stages by the council.
Site assembly will start in January next year, followed by demolition towards the end of 2001, with clearance completed by 2004.
A Southwark council spokesperson explained the length of the project timetable by saying that short-term planning led to a "mess". "We are taking a really long-term view," she said.
She added that the competition judges, made up of residents, businesses and council members, were looking for vision. "The council is looking for creative ideas rather than the bidders just second-guessing what it wants," she said.
