The Ministry of Defence (MoD) has agreed to the £350m sale of Devonport Dockyard to Babcock International.
DML has a workforce of nearly 5,000
The defence group is buying Devonport Management Limited (DML) from a consortium including Kellogg Brown and Root and Balfour Beatty.
The MoD said the transfer of ownership was subject to regulatory approvals.
Defence Minister, Lord Drayson, said: "This is a significant step forward." The Ministry will however retain a share in the business.
Whilst the sale was agreed in May, this new announcement has secured the transfer of ownership from DML to Babcock International.
Devonport maintains, upgrades and fuels the Royal Navy's submarines, and the deal is set to make Babcock the UK's leading naval maintenance firm.
DML has controlled the Plymouth dockyard since 1997.
The firm has a workforce of nearly 5,000.