Campaigners say they will take legal action over a controversial speed limit on Windermere in the Lake District.
The 10mph speed limit came into force in March 2005
The Lake District National Park Authority (LDNPA) outlawed high-speed craft in March 2005 in a move to restore the area's tranquillity.
But after Keep Windermere Alive threatened a legal challenge the authority took advice from lawyers.
On Wednesday the LDNPA said it would not alter the limit and campaigners say they will now take the case to court.
After earlier discussions between the two sides, the authority was asked to consider opening talks on a managed solution, allowing a limited return to skiing and power boating.
The campaigners had been hoping for a compromise on the 10mph limit, and wanted to introduce competency tests to help ensure motor boat users were not a menace.
Kevan Furber, Secretary of Keep Windermere Alive, expressed disappointment following Wednesday's decision.
He said: "We had expected them to have more sense, but we haven't come this far for nothing.
"Windermere doesn't belong to the LDNPA board, it belongs to the country and there has been no real public consultation.
"We intend to legally challenge the bylaw, and will take our campaign to the High Court if necessary."
According to the LDNPA, a managed solution would be complex, costly and bureaucratic, and would create an uncertain future for Windermere.
On Wednesday a spokesman said: "Two years after introducing a 10mph speed limit on Windermere, members of the LDNPA today turned down a request to look at a managed solution aimed at bringing back jet skis, water skiing and power boating on England's longest lake.
"Expert legal opinion was given to members and explained that Keep Windermere Alive's challenge was unlikely to succeed."
The authority also said that a managed solution could not be reconciled with "the right of navigation", which allows users to take a boat onto the water at any time.
So far Windermere businessman Nick Fieldhouse is the only person convicted of exceeding the limit. He was fined £200 by Kendal magistrates in October 2005.