The government has published a review, instigated by the Lib Dems, on "alternatives" to the Trident nuclear weapons system.
The UK's existing four ballistic missile submarines have to be renewed from the late 2020s and a final decision is due in 2016.
Here is a summary of the main points.
- Nuclear weapons will remain a "political tool of last resort" and be considered only "in the most extreme circumstances"
- Their rationale is to deter extreme threats to the UK or to prevent an escalation that may risk "national survival"
- The credibility of any weapons system is "crucial" but it must also be safe and secure and affordable
- UK not able to conduct live tests of new warheads as signatory to Nuclear Test Ban Treaty
- Modifying civilian aircraft to carry nuclear missiles "very high risk"
- Stealth bomber expensive and would need support aircraft
- Low orbit vehicles "too risky" for conversion to nuclear platforms
- Mobile land-based missile launchers hard to protect and "operationally risky"
- Missiles based at fixed silo locations "more vulnerable" to pre-emptive attack
- Reconfiguring ballistic missile submarines to carry cruise missiles "discounted on cost grounds"
- New "dual use" submarines could be built but fleet size would have to be expanded
- Subsonic cruise missile potentially deliverable by 2035 despite technical challenges
- Supersonic cruise missiles "vulnerable" as may have to be located too close to adversary
- Hypersonic cruise missiles involve "extreme technical challenge" and multiplicity of platforms
- Hyper glide vehicles too much of a "technical challenge"
- Shorter range of cruise missiles could lead to "significant constraints" on UK options
- Ballistic missiles could potentially target any country due to long range
- Cruise missile warhead twice as expensive as ballistic one - at up to £10bn
- Length of time needed to develop cruise warhead and delivery system means two new ballistic submarines would have to be built in the interim to prevent a capability gap
- Report distinguishes between high, medium and low readiness
- Credibility based on readiness, reach, survivability and resolve
- Only a fleet of four ballistic missile submarines and missiles housed in silos could provide 30-year continuous deterrent
- For other platforms it would be "prohibitively expensive"
- Cruise missile submarines more "vulnerable" to attack while on operations
- Only a sophisticated adversary can "locate, track and engage" ballistic nuclear submarines
- Three boats would not be sufficient to maintain continuous patrols and could lead to "multiple unplanned breaks"
- Less regular patrols could be maintained with three or two boats based on Royal Navy priorities
- Any change may have wider implications for UK strategic interests
- Reduced capability may increase reliance on allies for nuclear security
- Cruise missile based system would be seen as "less capable" than a ballistic system
- Increasing readiness from a non-continuous posture might lead to "misunderstanding"
- Perceptions of the UK's commitment to multilateral nuclear disarmament may be increased
- Any new system would have to comply with treaty obligations
- The reliability of estimates varies widely due to the long time period involved
- Cruise missile platforms generally cheaper than ballistic ones
- But added expenditure on missiles and warheads mean keeping ballistic submarines more cost-effective
- Building three rather than four new Trident submarines would save money but only between 2025 and 2035