FIVE times a day, Muslims around the world turn to face Mecca in prayer. At least once in their lives, Muslims are also urged to join the annual pilgrimage to Islam's holiest city. This year, as in years gone by, millions of Muslims flocked to Mecca at the beginning of December on the haj, crowding its streets and filling the central Haram mosque to bursting. The city has always struggled to deal with this massive influx of visitors and there is now talk of redesigning the mosque to increase its capacity.

At present the Haram mosque can hold up to 900,000 worshippers. The new plans envisage creating space for 1.5m people in the main part of the complex, with the intention of expanding capacity still further in the future to allow up to 3m people to congregate there.

Norman Foster and Zaha Hadid, two respected and influential British architects, are among those who have apparently been approached to take on the project by King Abdullah, Saudi Arabia's monarch whose list of titles also includes “custodian of the two holy mosques”. Their involvement has raised hackles in Saudi Arabia.

Lord Foster may have won the prestigious Pritzker architecture prize but he is not a Muslim. The idea of a non-Muslim redesigning one of the world's most important mosques has provoked deep unease in Saudi Arabia. And as only Muslims are permitted to enter Islam's holiest city, were he to win the commission, Lord Foster would have the tricky task of seeing through his design from a distance.

This would not be the first time that foreign firms have been involved in sensitive building projects in the Middle East. Ten years ago, United Automation, an American firm, was given the job of redesigning the sound system in Mecca's central mosque. The entire system was successfully planned and built without those in charge in attendance. But rebuilding large sections of the world's biggest mosque without ever setting foot inside would be quite a proposition. Sami Angawi, a Saudi architect who founded the Haj Research Centre, which aims to preserve the history and architecture of Mecca and Medina, Islam's second city, is apprehensive about the plans, reasonably pointing out that in order to design a mosque, you have to visit it. Mr Angawi has also expressed his concern at the apparent lack of Saudi involvement.

Even before the plans to give the Haram mosque a facelift emerged, many Muslims were uneasy about the renovations already underway in Mecca. The modern city bears little resemblance to the birthplace of the prophet Muhammad. Visitors to Mecca can buy a latte from Starbucks and a snack from KFC or McDonald's. Moreover, the first Islamic school where Muhammad is believed to have taught as well as the house of Khadija, his first wife, are believed to have been destroyed as construction in Mecca has boomed. Critics such as Mr Angawi fear that if these plans go ahead, more damage will be wrought upon Mecca's historic buildings. Some suggest that Saudi Arabia's rulers don't concern themselves much about preserving these historic sites because their interpretation of Islam regards venerating holy places as akin to idol worship.

The proliferation of global chains, high-rise apartment blocks and five-star hotels has already raised fears that Saudi Arabia's rulers are less worried about the welfare of the millions of pilgrims who voyage to Mecca every year than they are about squeezing as much money out of them as possible. Tourism is a valuable component of the Saudi economy and of the 9m visitors each year some 2.5m come for the haj.

Taking part in the haj is a precarious business. People are often killed in the crush as hundreds of thousands of pilgrims jostle their way through tunnels and over bridges to complete the various rituals that form the pilgrimage. And as travelling to Saudi Arabia has become easier, more and more people are making the journey. The Saudi authorities have come under considerable criticism in the past for failing to prevent the stampedes in which many have died.

The Saudi rulers are doubtless well aware of the sensitivities involved in the attempts to renovate Mecca. And Lord Foster and Ms Hadid are used to criticism of their uncompromising modern architectural style. But if they do end up revamping Mecca, they should be prepared for a particularly hostile reaction.