Project manager Liam McGowan said the decision was made once the firms were in a two-horse race.
He said: "It was felt that you could leave the weaknesses of both teams and keep the strengths. We are delighted with the result." Others originally vying for the scheme included Alsop Architects/WS Atkins, Swanke Hayden Connell, Arup, Parkman/Andrew Martin Associates and Llewelyn-Davies.
The site, on 288 acres, includes the massive Ferrier housing estate, with about 2000 houses, the Kidbrooke school, sports centres, a shopping centre and a train station.
McGowan said: "We want to ensure that we articulate all the objectives of the stakeholders in the project into something that resembles a masterplan. It's quite a huge project." The site has been identified as a key area along the Thames Gateway. The brief for the area is to introduce higher density housing and a coherent transport interchange.
Mott MacDonald will handle the project management and engineering and EDAW will draw up the masterplan. Others in the team include Bovis Lend Lease, Gardiner & Theobald, Pricewaterhouse Coopers, HTA Architects and Dutch designer EEA Architects.