Technical issues and personnel shortages could delay the deployment of the Royal Navy's new aircraft carriers, the spending watchdog has warned.
HMS Queen Elizabeth and Prince Of Wales will provide the Navy with a capacity it will have been without for a decade.
But the National Audit Office says the project is entering a "critical phase", with many risks to manage.
The Ministry of Defence acknowledged "challenges" but said it was committed to being fully operational by 2026.
The current cost estimates for both carriers, as well as the Lockheed Martin F-35B jets that will fly off them, is £14.3bn, the NAO said in its report.
The NAO said technical problems means sea trials for the first carrier, HMS Queen Elizabeth, will not now take place until the summer - three months later than planned.
It added that further technical issues could mean the carrier will not be operational by 2020, as has been promised by the MoD.
Among the other areas mentioned by the NAO were:
- Personnel issues, in particular a shortage of engineers for both warships. The report says while the number of pilots needed is "just sufficient" there could be problems if personnel left the forces. Pilots for the new F-35B jets need four years fast jet training with an additional nine-month carrier conversion course, meaning they will not be easy to replace
- Timing: The next three years will be crucial, the report says, as the carrier brings together the jets, helicopter and radar as well the crew and the ships' support. The NAO says there is "no further room for slippage". And there are still technical risks, not least with the F-35B jets still being designed and tested
- Cost increases: The NAO says the MoD is already facing a cost overrun of up to 2% on the £6.2bn budget for building the two ships. Costs for buying the F-35B jets could rise because of currency fluctuations.
The NAO said the introduction of the carriers will also have a far-reaching impact on the way the Royal Navy operates.
A "significant proportion" of its fleet will be required to support and protect the ships, with a carrier task group likely to account for around 30% of the total fleet and 20% of the Navy's personnel.
In a statement, the Ministry of Defence said it had the personnel needed for HMS Queen Elizabeth and plans for manning HMS Prince of Wales were "advanced".
A spokesman added: "With sea trials expected to start in the summer, we recognise that there are challenges ahead and remain committed to delivering the full range of joint F-35 and carrier operations by 2026."