London, Oct 28 (IANS): Saudi Arabia will build a mosque so big that it can hold up to 1.6 million worshippers, The Independent reported Saturday.

But to build that they are also demolishing irreplaceable monuments.

Three of the world's oldest mosques are about to be destroyed as Saudi Arabia embarks on a multi-billion-pound expansion of Islam's second holiest site.

Work on the Masjid an-Nabawi in Medina, where the Prophet Mohamed is buried, will start once the annual Hajj pilgrimage ends next month, the newspaper said.

When complete, the development will turn the mosque into the world's largest building, with the capacity for 1.6 million worshippers.

But concerns have been raised by academics that the development will see key historic sites bulldozed.

The Saudis have announced no plans to preserve or move the three mosques, which have existed since the seventh century and are covered by Ottoman-era structures.

With cheap air travel and booming middle classes in populous Muslim countries within the developing world, both Mecca and Medina are struggling to cope with the 12 million pilgrims who visit each year - a number expected to grow to 17 million by 2025.