WORK began today on a major research excavation to investigate the bluestones at Stonehenge.

English Heritage has agreed to the excavation following the granting of Scheduled Monument Consent by the Secretary of State for Culture, Media and Sport.

The last time an excavation was allowed inside the stone circle was in 1964.

The excavation at Stonehenge will last for two weeks until April 11.

During this time Stonehenge will be open as normal and visitors will be able to observe up close the excavation as it happens on plasma screens inside a special marquee.

The excavation, led by renowned Stonehenge academics Professor Tim Darvill of University of Bournemouth and Professor Geoffrey Wainwright, President of the Society of Antiquaries, aims to provide a more precise dating of the Double Bluestone Circle, the first stone structure that was built on the site.

A trench measuring around 3.5 metres by 2.5 metres will be dug in a previously excavated area on the south-eastern quadrant of the Double Stone Circle with the hope of retrieving fragments of the original bluestone pillars.

Samples obtained from this excavation will be tested using more advanced technology such as radiocarbon dating and will throw light on how long the Circle was in use for, when it was dismantled and reused in later stages of Stonehenge's construction.

Dr Simon Thurley, chief executive of English Heritage, said: "The bluestones hold the key to understanding the purpose and meaning of Stonehenge.

"Their arrival marked a turning point in the history of Stonehenge, changing the site from being a fairly standard formative henge with timber structures and occasional use for burial, to the complex stone structure whose remains dominate the site today.

"English Heritage has a duty to encourage the best research on historic properties under our care. This is a tremendously exciting piece of research that will help us find out considerably more about the important questions concerning the bluestones and I look forward to the results of their work."

BBC Timewatch in association with Smithsonian Networks will fund the excavation and post excavation analysis and will also film it for broadcast on BBC2 in the autumn.