Now confirmed by Philip Hammond:
Underlying facts upon which the earlier decision was made have changed. 2023 was the earliest at which EMALS could be fitted - the estimated cost has more than doubled in the last 17 months with possibility of further increases.
F35 STOVL programme has made excellent progress in recent months; the balance of risk has changed, so that F35B poses no greater development risk than other JSF variants.
'Cats and traps on a single carrier was the right decision on facts made at the time'. However, the delay in the programme for 2020 was unacceptable.
Both carriers will now be completed to operate STOVL versions of the F35; order for JSF will now be changed to F35B. First production F35B should be delivered in 2016, to fly from HM Queen Elizabeth in 2018 after her sea trials in 2017. Force to be operational in 2020.