On 29 March 2026 the Defence Equipment & Support (DE&S) organisation of the UK Ministry of Defence (MoD) made a ‘Proud moment alert’ on its Facebook page, announcing: “We’ve accepted the final Tranche 1 F-35B Lightning, completing delivery of the UK’s first batch of the world’s most advanced fighter jet.”

This was ‘half true’ and may have been accurate in a narrowly legalistic sense, but it left an entirely misleading impression. Some reading the announcement would assume that all 48 of the F-35B Lightning short-take-off/vertical landing (STOVL) Joint Strike Fighters in the UK’s first tranche were now in UK hands – a few even remembering that one aircraft had been written off after an accident on HMS Queen Elizabeth in November 2021. In fact, two aircraft were stranded on the Azores mid-way through their planned delivery flight and three more remain at Lockheed Martin’s Fort Worth factory, ‘accepted’ by the MoD – and presumably triggering payments to Lockheed Martin – but not yet delivered.

The last three aircraft were all ‘accepted’ on the day they made their maiden flights (or the following day), perhaps implying that this was a milestone that has to be achieved before an aircraft can be accepted, maybe without requiring post-test rectification or essential follow-up flights to have been ‘squared away’.

Allowing for the loss and the non-delivered aircraft, one might assume that there would be 42 F-35Bs at RAF Marham, equipping the two frontline squadrons (No 617 Squadron ‘Dambusters’ and No 809 Squadron) as well as the training unit, No 207 Squadron.

However, six aircraft are assigned to the operational evaluation unit: No 17 Test and Evaluation Squadron (17 TES), operating from Edwards Air Force Base (AFB), Naval Air Station (NAS) Patuxent River and Marine Corps Air Station (MCAS) Beaufort. One is at MCAS Cherry Point for upgrade, while four are at the Fleet Readiness Center South East at NAS Jacksonville for Production Asset Inspection Requirement Tier II (PAIR II) corrosion-mitigation inspections. This is a response to what the NAO called “an emerging issue with higher corrosion than expected in maritime environments”.

This leaves 23 aircraft at RAF Marham and eight forward deployed to RAF Akrotiri on Cyprus.

Nor are all aircraft to the same standard. Five Marham aircraft (and three at Edwards AFB) are early low-rate initial production (LRIP) jets, originally delivered with Technology Refresh Level 1 (TR1) software and believed to be incapable of being upgraded to TR-3 standards (the entry point and enabler for the coming Block 4 upgrade).

Six aircraft at Marham (and two aircraft assigned to 17 TES) were built with TR3 software and six more (one assigned to 17 TES) have been upgraded to the same standard. Unfortunately, the TR3 programme has been dogged by delays and difficulties, with the initial ‘truncated’ software release (40P01) providing just an “initial training capability”, while the second (40P02) confers only a “more robust combat training capability”. Neither makes the aircraft fully ‘combat-coded’, however, meaning that about 11 UK Lightning Force aircraft are essentially capable of ‘training missions only’.

This means that the UK Lightning Force has about 16 aircraft with TR2 software and five built with TR1. These are believed to be the only fully operational F-35Bs at the moment, though TR1, TR2 and TR3 aircraft all participated in last year’s Carrier Strike Group deployment aboard the aircraft carrier HMS Prince of Wales.

The Block 4 upgrade is not fully defined and has been badly delayed, but does promise to increase F-35 capabilities, not least through the integration of the MBDA Meteor beyond-visual-range air-to-air missile (BVRAAM) and the Spear air-to-surface missile. In 2013 the MoD expected that Block 4 would be fully delivered by 2022 and required Meteor and Spear by 2024, but by 2023 the upgrade was reported to have been delayed until 2029 and the F-35 Joint Program Office (JPO) now expects that Block 4 will not be completely delivered until 2033, with Meteor and Spear entering service “in the mid-2030s”. The MoD now wants to mitigate capability risk by purchasing interim weapons.

A UK National Audit Office (NAO) report on 11 July 2025 revealed that the UK F-35 fleet was available to perform all required missions (the full mission-capable rate) for about one third of the MoD’s target. This represented two-fifths of the global F-35B fleet’s full mission-capable rate. It also revealed that availability to perform at least one of seven possible required missions (a mission-capable rate) was about half of the MoD’s target figure (and three fifths of the global fleet rate). The NAO did not disclose the actual percentages “for national security reasons”. It was stated that UK rates were significantly lower than those of the global F-35B fleet.

This has had an impact on training and the Royal Air Force (RAF) has been unable to reach the modest MoD requirement of flying 10 hours per pilot per month, nor even the reduced requirement of approximately 7.5 hours per month. The RAF has also been operating fewer than the budgeted synthetic training hours.

Given the poor availability of the F-35B, this means that a much smaller number of aircraft would be expected to be ready for operations at any time and that pilots might be inadequately prepared for complex missions.

The programme has cost more than was anticipated it would spend at the time of the 2013 business case. The NAO has estimated that the MoD has spent GBP 11 billion (EUR 12.63 billion) to date on F-35 capability, with an additional GBP 1.5 billion on personnel, infrastructure, training flights, other overheads and earlier sunk costs. The NAO judged that: “In our view, the capability achieved for the estimated GBP 11 billion spent to date is a disappointing return so far.”

The NAO said that the F-35B programme’s whole-life costs are considerably higher than the MoD has publicly reported. A public forecast for whole-life costs of GBP 18.76 billion assumes that only 48 aircraft will be procured and that these will leave service in 2048.

In June 2025 the UK’s Strategic Defence Review (SDR) stated that the UK would require more F-35 aircraft over the coming decade and subsequently announced that it would procure another 27 aircraft between 2030 and 2033, including 15 more STOVL F-35Bs and 12 conventional take-off and landing F-35As. The latter were justified on the basis of their lower unit price and longer endurance for conversion training, but also due to the fact that they would be capable of carrying US-owned nuclear weapons under the NATO Dual Capable Aircraft (DCA) nuclear sharing arrangement.

Official MoD policy is still to procure an eventual total of 138 aircraft with an out-of-service date of 2069. This would result in whole-life equipment and support costs of GBP 57 billion – or GBP 71 billion including wider MoD costs such as personnel, fuel and infrastructure.

The UK requirement for 138 aircraft was used to determine the composite share ratio (CSR) paid to the programme, covering each customer nation’s share of the global project administration, sustainment, upgrade and overhead costs. As a result, the UK’s CSR is 4.6%: the second highest in the programme after the US contribution of 81.9%.

The UK was the first partner to join the US in the Joint Strike Fighter project in 1995 and paid 10% of the concept demonstration phase costs (GBP 144 million). In January 2001 the UK committed a further GBP 1.7 billion (USD 2 billion) to become the only ‘Level 1’ partner for the F-35’s system development and demonstration phase to 2020. This Level 1 status allowed input into the design and development of the aircraft and was supposed to guarantee some ‘freedom of action’ in terms of upgrades, support and sustainment.

The upbeat DE&S announcement noted that: “Flying from our Queen Elizabeth-class carriers, these aircraft are a powerful symbol of UK defence capability, today and for decades to come.” Critics suggest that the F-35B may well be symbolic of UK defence capability, in that the true situation is rather less impressive than a superficial impression might suggest. However, whatever the F-35B’s problems, it has been acknowledged that: “The capability provided by the F-35 across its multiple roles is significantly superior to any previous UK aircraft.”