The Remotely Piloted Aircraft System (RPAS) will provide "critical" surveillance capability, according to the Ministry of Defence (MOD), and carry precision strike weapons.
They are capable of operating anywhere in the world while being controlled from the UK.
Sixteen of the aircraft have now been ordered from General Atomics Aeronautical Systems Inc (GA-ASI) and are expected to enter service with the RAF by mid-2024.
The Protector uses a design built upon the existing Reaper drone system which it is replacing.
It is designed to reach heights of 40,000ft with a payload of up to 4,800Ibs and is capable of flying consistently for up to 40 hours.
Protector Programme Senior Responsible Owner, Air Commodore Richard Barrow, said: "The contract for the additional 13 Protector aircraft, taking the total to 16, is a major milestone for the UK.
"When Protector enters service in 2024, UK defence will take an enormous jump forward in capability, giving us the ability to operate globally with this cutting-edge, highly-adaptable platform."
The first three Protectors were ordered last year, with the first two currently undergoing tests in the US.
The contract for the first three Protectors included an option for the 13 remaining aircraft.
According to the MOD, the Protector could operate in civilian air space and, if requested, be able to support with civilian search and rescue and disaster response missions.
The first Protector is expected to be delivered to the RAF in 2023. They will be based at RAF Waddington in Lincolnshire.
A pre-production example of the Protector is set to operate from Waddington later this year before taking part in the multinational large-scale Exercise Joint Warrior in Scotland.
Watch: Forces News was given special access inside Reaper Force at RAF Waddington in 2019.
The Protector will be fitted with the Brimstone missiles and Paveway bomb systems and will use onboard sensors to avoid other aircraft.
It will have seven weapons stations – meaning a total of 21 Brimstones could be carried – which is significantly more than the Reaper.
The £195m contract was announced by Minister for Defence Procurement Jeremy Quin during a visit to GKN Aerospace, which is making parts of the aircraft on the Isle of Wight.
The investment also includes four additional ground control stations and support equipment.
Cover image: RAF Protector drone on its first flight last year (Picture: MOD).