The Defence Command Paper has been published, giving more information on the military's future following last week's Integrated Review.
The document goes into greater detail around how the British Army, Royal Air Force and Royal Navy will look in the coming years.
To coincide with its publication, Defence Secretary Ben Wallace gave details to the House of Commons.
The number of personnel in the Army will be reduced to 72,500 by 2025.
A target of 2025 has been set for soldier numbers to be decreased, with a focus being put on developing the role of reservists.
Defence Secretary Ben Wallace has said the reduction will "not require redundancies", echoing comments made by the Prime Minister last week, and will not result in cap badges being lost.
In November 2019, Boris Johnson pledged that: "We will not be cutting our armed services in any form."
According to the latest data, the service currently has just over 103,000 trained personnel in total, including just over 76,000 regular soldiers – it represents a shortfall based on the total target of 112,000 personnel for the entire Army.
The 2015 National Security Strategy and Strategic Defence and Security Review (SDSR) stated ambitions "to not reduce the [regular] Army to below 82,000".
The numbers of British Army reservists will remain at about 30,000.
The remaining Challenger 2 fleet will be retired, and the Warrior will be replaced by the mid-2020s by the Boxer, with planned upgrades being cancelled.
The Defence Secretary said £23 billion will be spent on modernising the UK's land forces over the next four years.
The service's new Ranger Regiment, which will be established by August, will be seeded from 1st Battalion Royal Scots, 2nd Battalion Prince of Wales' Royal Regiment, 2nd Battalion Duke of Lancaster's Regiment and 4 RIFLES.
It will be a major part of the new special operations brigade, which the Army hopes to be able to deploy by 2022.
The Army will reorganise into seven brigade combat teams, including: two heavy, one deep strike, one air manoeuvre, and two light, plus a combat aviation brigade.
16 Air Assault Brigade will gain an additional unit, and will help form a global response force.
The Mercian Regiment's 1st and 2nd Battalions will merge, forming a new Boxer mounted battalion.
As part of plans for the new infantry, regiments will sit in four divisions, each with a "balanced number of battalions", giving personnel a "wider range of choices and opportunities".
"Intelligent recruiting for the infantry" will be introduced, initially feeding the four Ranger Regiment battalions.
The Army's oldest CH-47 Chinooks will be retired, with newer variants of the helicopter coming in.
The service's Pumas will be replaced, while the AH-64 Apaches will be upgraded by 2025, with a new 'medium-lift' helicopter being utilised to turn the capability from four types of aircraft to one.
Watchkeeper will be upgraded also.
Details on wider Army restructuring will be announced by summer 2021.
Royal Air Force
On the UK's F-35B aircraft, Ben Wallace has committed to "grow the fleet beyond 48 aircraft".
There was a pledge in the Integrated Review for the UK to have "at least 48 F-35s by 2025", while in September 2020, an overall plan for the UK to eventually own 138 of the aircraft was described as the "upper limit" of how many would be bought.
Nine Reaper drones, used to conduct airstrikes as part of Operation Shader, will be replaced by the Protector by 2024.
The C-130J Hercules aircraft will be retired in 2023, with the capability transitioned to the C-17 and 22 A400M aircraft for a "more flexible transport fleet".
The tranche one Typhoons will be retired in 2025, although the RAF will retain seven Typhoon squadrons, with its radar being updated.
The Hawk T1, a training aircraft used by RAF and Royal Navy front-line units, is also being retired.
Over the next four years, £2bn will be invested in the Tempest programme, which plans for a sixth-generation fighter jet by 2035, as well as developing the LANCA (Lightweight Affordable Novel Combat Aircraft) capability.
The aim is for the UK to have more than 20 destroyers and frigates by 2030.
New automated minehunting systems will be deployed in the future, replacing the Sandown and Hunt classes, which will both retire in the 2020s.
The Harpoon missile will be replaced by the interim Surface to Surface Guided Weapon, with upgrades being given to air defence weapon systems on Type 45 destroyers, vessels which will be replaced by Type 83s from the late 2030s.
Development will also be carried out on the next generation of subsea systems for the 2040s.
Two new types of submarine will enter service, including Dreadnought-class ballistic missile vessels to replace the Vanguard-class.
The service will also get three new classes of support vessels, including a "new Multi-Role Ocean Surveillance Ship".
The Royal Marines will also see their way of working evolve, with their personnel taking on special operations, as part of the Future Commando Force (FCF).
There will be a £40m investment to develop the FCF and transform British amphibious forces, with more than £50m spent on converting a Bay-class support ship for "a more agile and lethal littoral strike capability".