The MoD has announced a £258 million contract with helicopter firm AgustaWestland that it said would support 500 UK jobs.
The deal is to provide technical and maintenance support for the 90-strong Sea King fleet used by the Royal Navy and Royal Air Force for the next three years.
It is an extension of an arrangement that has been in place with Yeovil-headquartered AgustaWestland since 2005.
After this final three-year phase, Sea Kings are to be retired from service and replaced with Merlin helicopters.
Sea Kings, which have been in use since 1969, are deployed in search and rescue services in the UK and also sometimes to carry troops and conduct surveillance.
Defence Equipment Minister Philip Dunne said: "The Sea King helicopter fleet has been in service for over 40 years providing a vital capability in air/sea rescue and surveillance.
"This maintenance agreement, securing 500 jobs across the UK, demonstrates our commitment to keeping this aircraft in top condition until it is retired from service."
Royal Navy Captain Tony Gray, Defence Equipment and Support Sea King team leader, said: "Securing this contract ensures value for money for the taxpayer while maintaining the provision of search and rescue helicopters around the UK, 24 hours a day throughout the year."