The Ministry of Defence has objected to plans to build a zip wire across Thirlmere.
In a letter to the Lake District National Park Authority, the MoD said the proposals pose a risk to low flying operations, as military aircraft are unable to identify and avoid wires.
The Cumbrian firm Treetop Trek wants to create a new activity hub in the national park.
A public consultation on the plans ended last week.
Treetop Trek says it's surprised by the MoD's stance: