Northern Ireland's richest man claims no legal authority existed for declaring a public right of way on his land, the High Court heard today.
Counsel for Lord Ballyedmond said new concerns had emerged under Freedom of Information legislation.
But Newry and Mourne District Council lawyers argued he should not be allowed to make eleventh-hour changes to his case.
The dispute over a laneway on the industrialist's estate, beside Killowen Beach in Co Down, is set to go to trial next week.
Lord Ballyedmond, also known as Dr Edward Haughey, is head of leading veterinary pharmaceuticals company Norbrook Laboratories.
He is seeking a ruling that the stretch of land is private.
The council, however, wants a determination that the route should be open to the public.
Lord Ballyedmond has already secured an injunction preventing access to the laneway.
It was granted after concerns were raised about any alleged trespassers coming close to sensitive research facilities.
Ahead of a court battle listed for eight days Lord Ballyedmond's legal team sought permission to amend their case based on the Freedom of Information findings.
Stephen Shaw QC said it was being contended that a council sub-committee lacked evidence and went outside its legal constraints in asserting a public right of way.
Paul McLaughlin, for Newry and Mourne, claimed the new argument should be ruled out due to delay.
He further contended that it amounted to an "inappropriate attack" on the council's decision.
"I have described it as a collateral public law challenge," he told the court.
"What the plaintiff says is the sub-committee of the council which made the assertion did not have the legal authority to do so."
He insisted that even if "some procedural irregularity" existed, the decision should stand on the principles of good administration.
Mr McLaughlin added: "What we are dealing with here is a public right of way.
"The council only asserted it. It didn't assume it for itself."
But according to Mr Shaw the issues should be debated fully at trial.
Replying to a point raised by the judge about the time spent on the case by council officials, He stressed: "We have always been seeking a declaration that the lanes are private and not subject to any public right of way."
The case continues.