A £10m marina scheme in Fishguard may be delayed as a businessman is claiming ancient rights to the ownership of a large section of Pembrokeshire coast.
Lawyers are examining Mark Roberts' claim to a 45-mile stretch between Newgale and Fishguard after acquiring the title Lord Marcher of St Davids.
It was thought the land belonged to the Crown Estate which was looking to lease it to allow the marina to proceed.
Mr Roberts said he has his own ideas for the marina if his claim is upheld.
Law lords last year quashed his efforts to charge some families in Peterstone, near Newport, Gwent, thousands of pounds to use their own drives which crossed common land that he owned.
The Crown Estate, which owns and manages huge areas of the UK, said it was currently examining Mr Roberts' claim.
It owns just over half of the UK's foreshore with the rest in private, local government or conservation hands.
It wants to lease the foreshore at Fishguard to Pembrokeshire Council to allow the National assembly backed marina development to proceed.
Mark Roberts claims the land as Lord Marcher of St David's
But Mr Roberts, who cautioned the Land Registry in 2003 that he was the rightful owner, said: "This is not some sort of spurious claim, we are the true owners of the land."
He said he would not jeopardise the marina project but said he would want assurances it would benefit 'the indigenous population', jobs created would go to local people and there would be a residency qualification for some of the moorings.
The dispute over who owns the land could run for months or years.
Pembrokeshire Council said the marina project was central to its regeneration plans for that area.
But the authority and its councillors are not commenting on the matter as they anticipate being drawn into the dispute.
"I hope he would ensure there were mooring rights for local people"
Opinion in Fishguard is divided. One resident said: "I think if there is big claims for the rights of the land then it will take a long time and go court and that will be the end of the marina.
But another added: "If Mr Roberts really owns the land I hope he would ensure mooring rights and jobs for local people."
A few of the families involved in the land row at Peterstone did pay Mr Roberts' company Trelleck Estate over the access issue.
Newport West MP Paul Flynn said "He (Mr Roberts) caused a lot of heartache and anxiety to people."