A team of Nigerian and British archaeologists say the wall, which could be more than 1,000 years old, is genuine and not a modern construction.

Nigerian expert Levi Izuakor confirmed the wall was evidence of a great ancient city, but that little more than that could be said about its origins at this time.

The find had prompted suggestions that the centre of one of Africa's greatest kingdoms - and the final resting place of the Queen of Sheba - had been discovered.

But experts say the dating of the wall could eventually place its construction many hundreds of years after the lifetime of the Old Testament figure.

The wall lies hidden in the Nigerian rainforest at a site called Eredo, just a few hour's drive from the capital, Lagos.

A team from Bournemouth University, working with British archaeologist Dr Patrick Darling, recently completed a preliminary survey of the Eredo earthworks.

They consist of a wall and ditch measuring 70ft (20 metres) high in places and approximately 100 miles (150km) long.

'Earliest rainforest kingdom'

While not approaching the complexity of a project like the pyramids in Egypt, the builders would have shifted an estimated 3.5 million cubic metres of earth during construction of the ramparts.

This is one million cubic metres more than the amount of rock and earth used in the Great Pyramid at Giza.

Archaeologists believe it marks out what is believed to be the boundary of the Ijebu kingdom, ruled by the 'Awujale' spiritual leader.

Awujale: Local link to Sheba

The British eventually broke the Ijebu trade monopoly, leading to the decline of the kingdom.

However, the traditional position of Awujale still exists in the modern day town of Ijebu-Ode.

Dr Darling says the Eredo site is a particularly exciting discovery because it provides the "earliest proof of a kingdom founded in the African rainforest".

Monument of remembrance

People living nearby the Eredo monuments link the area to Bilikisu Sungbo, another name for Sheba, says Dr Darling.

Local tradition speaks of a great queen building a vast monument of remembrance, and there is an annual pilgrimage to what is believed to be her grave.

But scholars trace the legend of the Queen of Sheba to an Old Testament story, describing how the queen married Solomon and their son began a dynasty of rulers in Ethiopia.

Estimates suggest the Old Testament queen would have reigned in the 10th century BC - some 2,000 years before the Eredo wall is believed to have been built.

Dr Darling says the beliefs of the locals cannot be discounted.

"I don't want to overplay the Sheba theory, but ... the local people believe it and that's what is important."