Plans to extend the Lake District National Park 'should only go-ahead with residents' backing', according to Westmorland and Lonsdale MP Tim Farron.
The charity argues the move would better protect the landscape but Mr Farron says it could have a negative impact on people living in the area.
If accepted, the size of the Unesco site could be increased by around 155 square kilometres.
Local campaign group, The Southern Boundary Partnership, who are pushing for the bid say that 6% increase in area would benefit from the extra tourism.
The final decision will be made by Natural England and the Environment Secretary.