Two of Britain's most-renowned architects are in the running for the single most audacious renovation in history: the redevelopment of Mecca.

Norman Foster and Zaha Hadid are among 18 architects to have been approached about redesigning Islam's holiest city by building a mosque complex to host the three million Haj pilgrims who visit every year. The development would more than triple the central al-Haram mosque's current 900,000 capacity, making it the highest-occupancy building in the world.

The plans are thought to be backed by Saudi Arabia's King Abdullah bin Abdul Aziz. The remit is to "establish a new architectural vision" for Mecca's 356,800sq m mosques complex. The King is to be presented with the proposals by Hadid, 58, and Foster, 73, with those of the other designers at an exhibition at the end of the month.

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Sources close to the project told The Architect's Journal the scheme is likely to be phased, the first stage taking the al-Haram mosque capacity to 1.5 million. That would rise gradually until three million was reached. Neither Foster nor Hadid wanted to comment on the project last night. Hadid's spokesman said he "could neither confirm nor deny" speculation, while Foster's office said: "It has been leaked and not from us so I'm unable to comment."

Other sources describe the project as divided into two "tracks", one looking at various alternatives for the northern expansion of the al-Haram complex, and the other at the al-Haram itself. Lord Foster's firm, Foster & Partners, has been invited to partake in the former with 10 other firms including, reportedly, Atkins Design.

Zaha Hadid, with six other world-renowned firms, has been given the task of re-envisioning the al-Haram mosque itself, as well as "revisiting the whole area of the central district". British engineers Adams Kara Taylor and Faber Maunsell are also thought to be under consideration.