All submarines and almost all surface ships could be withdrawn from Devonport Naval Base under plans being considered by the government.
Nuclear missile submarines are refitted at Devonport
It is one of three options outlined in Westminster as part of the Ministry of Defence's (MoD) naval review.
Devonport currently employs about 2,500 civilian and service personnel and is home to 25 major ships.
Plymouth Sutton MP Linda Gilroy has warned the city would suffer badly from any cutbacks.
When the review was announced the defence secretary warned it could result in radical reductions in capacity.
Experts predict cutbacks would affect about 10,000 local jobs which depend directly on Devonport Dockyard and the £300m which it contributes to the local economy.
The review is said to have considered options from the status quo to closing all existing bases and building one big new one.
But on Tuesday Defence Minister Derek Twigg said the three core options now being considered involve withdrawing ships from Portsmouth, keeping all three bases at Faslane, Portsmouth and Devonport with cutbacks, or minimising Devonport with the withdrawal of all ships and submarines.
Plymouth's three MPs, Linda Gilroy, Alison Seabeck and Gary Streeter, have always maintained it was "vital" to retain Devonport's skills.
Linda Gilroy, who is one of the MPs on the House of Commons Defence Committee, said the dockyard was the biggest employer in the Devon and Cornwall sub-region and it was "extremely unlikely" the submarines would be withdraw.
A final decision is expected by the autumn.