Countryside protection campaigners are to challenge plans for two new housing developments at a public inquiry on Monday.
The homes are earmarked for the area between the A38 and A379
The Council for the Protection of Rural England warns of the "needless destruction" of Devon countryside if the go-ahead is given.
The plans are for 3,500 new dwellings at Sherford, South Hams and 2,900 houses, as well as employment development, at Broadclyst, near Exeter.
The Devon branch of CPRE wants the plans to be withdrawn and replaced by more
"Plymouth's housing needs are best met within the existing urban area, not outside it," said Jim Woolcombe, Chairman of CPRE Devon.
"Those promoting the new settlement at Sherford claim that it would help meet
Plymouth's housing shortage.
"Yet people want access to local services and employment and to retain links with the communities of which they are a part.
"It is impossible to ensure the new homes will go to local people.
"Rather than meet local need for affordable housing the new settlements are likely to fuel further migration into Devon."
Representatives of Plymouth's business community, including some developers,
have also expressed concerns about the potential impact a new development would
have on efforts to regenerate the city.
"Building a new settlement outside Plymouth will undoubtedly harm prospects for the city's regeneration by diverting resources,
investment and effort away from areas in Plymouth that need it most," said Mr Woolcombe.
"We would like to see constructive alternatives considered.
"These proposals miss the point, which is how to make Plymouth a place people want to live, work and invest in."
The CPRE said proposals for a new settlement in countryside at Broadclyst near
Exeter were equally worrying.
It would be a dormitory settlement isolated from Exeter's economic centre, fostering car dependency and increasing congestion.
The Council said it would do serious harm to east Devon's countryside and the quality of life of those who live and work there.