Alistair Erskine, who had decided to invest in 11,000 acres of land at Castle Grant, near Grantown-on-Spey, was disappointed by the bird population and believed he had been misled.
He launched legal action through the Scottish courts but was twice disappointed.
However, now the Supreme Court in London has ruled in his favour, and against the trustees of Viscount Reidhaven, including the current Earl of Seafield, Ian Derek Francis Ogilvie-Grant, who own the estate.
The case will return to the Court of Session to discuss damages, previously estimated at around £500,000, but now understood to be considerably higher.
The owners, who yesterday said they were “disappointed” by the judgment, recognised a need to increase the grouse population on the land by the river Spey in 2006, and sought a tenant.
In the early 1970s, the grouse moor had provided a substantial bag of grouse but, like other estates, productivity declined.
By the 1990s, the owners had put in place a recovery programme, with some success. The annual bag had gone up from 196 brace to 300 brace. The trustees decided to offer a 15-year lease of the moor to attract investment.
Mr Erskine showed an interest but was concerned about levels of shooting and whether there was an “adequate breeding population”.
However, he was reassured by an e-mail, written by the estate’s chief executive Sandy Lewis, which included details of grouse counts carried out earlier that year.
Crucially, the court found those figures to be misleading.
In the main judgment, Lord Reed said: “The areas of the moor in which the counts were carried out were not, however, representative of the moor as a whole, but were the parts of the moor which were considered to be the most heavily populated by grouse.
“As a result, the estimated grouse population, as stated in the e-mail, was well in excess of the actual population.”
Mr Erskine set up a partnership called Cramaso, with his wife, for the venture.
Previous rulings went against Cramaso because the managers of the estate had dealt with Mr Erskine, and were considered to have no duty of care to the new partnership.
But this line of reasoning was dismissed by the Supreme Court judges.
Lord Reed said: “A duty of care was also owed by the respondents to the appellant [Cramaso], when they negotiated and concluded the contract, on the basis of the discussions previously held with Mr Erskine.
“The respondents acted in breach of that duty of care, and are therefore liable in damages for any loss suffered by the appellant as a result.”
Mr Erskine welcomed the judgment.
“It is pleasing that the highest court in the land has recognised that Cramaso LLP was owed a duty of care in the context of this negligent misrepresentation,” he said.
In a statement, the trustees for the estate said: “The trustees are disappointed that the Supreme Court did not uphold all the previous judgments of the Court of Session.
“We will be deliberating the implications of the decision and cannot comment further at this stage.”