A shortage of key personnel could jeopardise the effective deployment of the Royal Navy's two new aircraft carriers.
The National Audit Office (NAO) said that the Carrier Strike programme was entering a "critical" phase with the first of the ships - HMS Queen Elizabeth.
The carriers arrival into Portsmouth has already been put back three months and the NAO warns further "technical issues" could mean the Ministry of Defence's plan for it to be operational by the end of 2020 is delayed.
Among the problems it highlighted was the shortage of engineers, intelligence personnel and warfighting specialists in the Navy and RAF air crew.
While the MoD has begun a recruitment programme to address the gaps, the NAO said the number of pilots was expected to be "just sufficient" to 2026, with "limited resilience" if personnel left the forces.
The MoD acknowledged that the carrier programme faced "challenges" but said it remained committed to getting both ships fully operational by 2026.
The NAO said the MoD was already facing a 1% to 2% cost overrun on the £6.2 billion budget for building the ships, while the forecast £5.8 billion earmarked for the US-built Lightning II fighters could be affected by fluctuations in the value of sterling.
In the longer term, the NAO said the deployment of the carriers would have far-reaching implications for the way the Navy operates with a "significant proportion" of the fleet required to support and protect them.
The formation of a carrier task group is likely to account for about 27% of the Navy's fleet by tonnage and 20% of the personnel needed to crew the fleet.