Defence Secretary Grant Shapps has ended speculation about the future of the Devonport-based warships HMS Albion and HMS Bulwark.
There had been reports the two assault ships would be scrapped or mothballed to free up sailors for other ships due to recruitment shortages.
But on a visit to Devonport naval base in Plymouth, Mr Shapps said they would remain in service for another decade.
"We are not scrapping these ships, we are not mothballing them," he said.
"We are keeping them available for our fantastic Royal Marines," he added.
Mr Shapps also announced a £1.85bn contract with Thales UK to upgrade sonar and sensors on Royal Navy vessels.
He said around 150 engineering jobs and apprenticeships would be created at Devonport, Faslane, Glasgow, Portsmouth, Cheadle, Crawley and Bristol, as well as sustaining 300 from a previous contract with Thales.
The defence secretary's visit follows an announcement in November of a £750m upgrade to facilities at Devonport where navy submarines are maintained.