The site was discovered by chance last year by oceanographers from India's National Institute of Ocean Technology conducting a survey of pollution.

Using sidescan sonar - which sends a beam of sound waves down to the bottom of the ocean they identified huge geometrical structures at a depth of 120ft.

Debris recovered from the site - including construction material, pottery, sections of walls, beads, sculpture and human bones and teeth has been carbon dated and found to be nearly 9,500 years old.

Lost civilisation

The city is believed to be even older than the ancient Harappan civilisation, which dates back around 4,000 years.

Marine archaeologists have used a technique known as sub-bottom profiling to show that the buildings remains stand on enormous foundations.

The whole model of the origins of civilisation will have to be remade from scratch

Graham Hancock

Author and film-maker Graham Hancock - who has written extensively on the uncovering of ancient civilisations - told BBC News Online that the evidence was compelling:

"The [oceanographers] found that they were dealing with two large blocks of apparently man made structures.

"Cities on this scale are not known in the archaeological record until roughly 4,500 years ago when the first big cities begin to appear in Mesopotamia.

"Nothing else on the scale of the underwater cities of Cambay is known. The first cities of the historical period are as far away from these cities as we are today from the pyramids of Egypt," he said.

Chronological problem

This, Mr Hancock told BBC News Online, could have massive repercussions for our view of the ancient world.

Harappan remains have been found in India and Pakistan

"There's a huge chronological problem in this discovery. It means that the whole model of the origins of civilisation with which archaeologists have been working will have to be remade from scratch," he said.

However, archaeologist Justin Morris from the British Museum said more work would need to be undertaken before the site could be categorically said to belong to a 9,000 year old civilisation.

"Culturally speaking, in that part of the world there were no civilisations prior to about 2,500 BC. What's happening before then mainly consisted of small, village settlements," he told BBC News Online.

Dr Morris added that artefacts from the site would need to be very carefully analysed, and pointed out that the C14 carbon dating process is not without its error margins.

It is believed that the area was submerged as ice caps melted at the end of the last ice age 9-10,000 years ago

Although the first signs of a significant find came eight months ago, exploring the area has been extremely difficult because the remains lie in highly treacherous waters, with strong currents and rip tides.

The Indian Minister for Human Resources and ocean development said a group had been formed to oversee further studies in the area.

"We have to find out what happened then ... where and how this civilisation vanished," he said.