A diver measures a section of the stem, believed to be of the bow

Divers have excavated a five-metre-long piece of wood, which they believe is the front stem of the vessel's keel.

Archaeologists believe this stem could be attached to the front section, the bowcastle, of the ship which sunk in 1545.

Experts hope the find is the missing piece of the jigsaw in the collection of artefacts from the vessel.

Chief executive of the Mary Rose Trust, John Lippiett, called the discovery: "An extraordinary and exciting find."

He said: "There is a chance that we have found the bowcastle of the Mary Rose and I am saying that because I am an optimist. What we have found is in remarkable condition."

It's a vulnerable site and we have to protect it. We have reburied it and we will now stop fishermen from getting at it. Chief Executive John Lippiett In pictures: searching the shipwreck

Over the past month divers have been looking for more pieces of the ship, most of which was raised from the murk of the Solent in 1982 after 437 years under the sea off Portsmouth.

The "find" now means that a complete cross section of the warship has now been discovered.

However, experts have no idea what the bowcastle actually looks like - the earliest picture of Henry VIII's flagship was painted in 1547, two years after the vessel sank.

Alex Hildred, the dive's project manager, said that if the find was confirmed it would be "the most important maritime archaeology find in England in the last 20 years".

THE HISTORY OF A WARSHIP Mary Rose sank off Portsmouth on 19 July, 1545, in front of Henry VIII, who was watching his favourite ship engage the French It appears she was overloaded or mishandled and not, as the French claim, holed by one of their cannon Built between 1509 and 1511, she was the pride of the Tudor fleet and was one of the first warships which could fire a broadside She was named after Princess Mary, Henry's youngest sister Searches for the ship began in the 1960s, but it was not until 1971 that she was found More than 60 million people worldwide watched the raising of Mary Rose live on television on 11 October 1982

The reinvestigation of the wreck site started after the Royal Navy revealed �200m was to be spent on regenerating Portsmouth so it can house the next generation of warships, including the new aircraft carriers.

However, the route to the naval base is not deep enough for the new super-carriers - which are twice the size of existing vessels.

The Navy's preferred route crosses the wreck site of the Mary Rose, a couple of miles out into the Solent from the harbour mouth, and will need to be dredged.

The Mary Rose was the pride of Henry VIII's navy when she sank in 1545.

Artefacts discovered

Three quarters of Henry VIII's flagship was raised in 1982 and marine archaeologists have been searching for the final quarter bowcastle.

Divers have been visiting the wreck for the first time since the main part of the ship was raised.

They have been aboard the dive ship Terschelling working about 15 metres down in poor visibility.

During the dive, archaeologists have discovered about 400 artefacts - 200 of which are likely to be Tudor, including coins, buckles and weaponry.

Any operation to raise and preserve the section is likely to run into tens of millions of pounds.

Mr Lippiett, added: "It's a vulnerable site and we have to protect it. We have reburied it and we will now stop fishermen from getting at it."