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.

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."

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