The Queen has won "squatter's rights" over large tracts of the Severn Estuary in a dispute with a controversial Cardiff businessman.
Mark Roberts owns 60 "Lord to the Manor" titles
Mark Roberts owns the paper title "Lord Marcher of Magor" and claimed it gave him rights to the land.
The area would be worth millions if tidal and wind power was set up there.
But High Court appeal judges in London said the Crown had been in unchallenged possession of the area for centuries and ownership must stay with the Queen.
The ruling is part of a complex case brought by Mark Roberts, after he tried to register ownership of land under the titles he bought between 1997 and 2003.
He brought the case under the name Mark Andrew Tudor, Lord Marcher of Trelleck, one of the 60 "Lords to the Manor" titles he owns.
In the past, he has used the Trelleck title to claim ownership of common land at Peterstone, Newport, and demanded some residents pay for access to their own homes.
However, the law was changed in 2005 abolishing charges for access across common land, as long as people can show they have been doing it for 20 years or more.
The dispute centred on who owned parts of the Severn Estuary
In the latest dispute with the Crown Estate Commissioners, who administer land owned in the name of the Queen, judges were forced to examine legislation dating back to the 13th Century at the time of the Magna Carta.
The titles bought by Mr Roberts often date back to after the Norman conquest, when estates of land known as manors where handed to nobility, often in return for military service.
The Lordship of Magor was acquired by Mr Roberts in 1997, and he argued it dated back to the conquest of Wales by Edward I in the 13th Century.
He insisted it gave him ownership of mudflats, sand and the river bed on the Welsh side of the Severn Estuary.
Mr Roberts disputed that the Crown could claim what is known as "adverse possession" over one of its own subjects - in effect claim squatter's rights.
Under the legislation, a person who can show they have held unchallenged possession of a piece of land or property for 12 years can claim it as their own.
However the appeal court judges, Lord Justice Mummery, Lord Justice Jacob and Mr Justice Mann dismissed Mr Roberts appeal, ruling "beyond doubt" that the same law of adverse possession "applies to both the Crown and citizen".
Mr Roberts now faces a substantial legal bill, after the court also ordered that he pay costs in the case, which will be assessed at a later date.