The government has given the clearest possible undertaking that it will not pull out of Devonport Naval Base.
Hundreds of jobs are to be shed at the dockyard over the next year
Defence Minister Bob Ainsworth said rumours and fears the base could close in five years were "completely without foundation".
The minister was responding to Plymouth MP Linda Gilroy, who also warned it was essential for Devonport to maintain a broad range of defence work.
Mr Ainsworth also told MPs that Flag Officer Training was safe in the city.
Devonport is the oldest naval base in Britain and is seen as vital to the region's economy.
Babcock Marine, which owns Devonport Dockyard, announced plans last month to axe about 600 jobs over the next year.