The trunk of a giant oak tree unearthed from a field in Britain may be more than 5,000 years old, forestry experts say.

They called the 44-foot Norfolk bog oak "the largest-ever intact 5,000-year-old sub-fossilized trunk of an ancient giant oak," but said they believe it could be just a section, possibly just a quarter, of the original tree.

Until about 7,000 years ago the East Anglia fenland basin, a naturally marshy region in eastern England, was densely forested by gigantic oak trees. That changed with a rise in sea level that caused rivers to back-up and flood the fens.

Trees died where they stood until eventually falling into the silt of the forest floor, which led to their preservation, experts said.

Planks cut from the Norfolk trunk will be dried over seven months in a special kiln and made into "a breathtaking table for public display giving an insight into the grandeur of these ancient giant forests," a spokesman said.

"This one is so special in that it is intact and, as far as I can tell, sound along its full 44-foot length," Hamish Low of furniture specialist firm Adamson and Low said.

One of the rarest forms of timber in England, when bog oak is dried, experts said, it is comparable to some of the world's most expensive tropical hardwoods.