Madonna's lawyers have questioned the legality of Britain's land classification at a public meeting.
The Agency is preparing for new legislation
The pop star wants to keep the public off her country estate on the Wiltshire-Dorset border.
She claims land on the 1,200-acre estate has been inaccurately marked as open country - and the public should not be allowed access.
David Elvin QC told the Shaftesbury hearing that the Agency's methodology to classify some land was "incomplete".
"It (the methodology) doesn't have the force of law. If you apply only the methodology you are not necessarily
acting lawfully," said Mr Elvin.
He had earlier argued that the land was farmland, as opposed to open countryside.
The Countryside Agency, which is mapping areas of England to implement the Countryside Rights of Way Act 2000, has classified around 100 acres of the
estate as "downland".
The public will have the right to walk over any land marked on the maps as open country - mountain, moor, heath, or down - or registered common land once the act is in force.
But Madonna and her film director husband, Guy Ritchie, are appealing against the Agency's decision at the hearing in the Royal Chase Hotel in Shaftesbury, Dorset.
Chris Smith, appeals adviser for the Agency, told the inquiry: "We think that's (the methodology) the sole piece of information that should be used to
map open country."