The National Audit Office said the deal was years behind schedule
The Ministry of Defence failed to get the best deal for taxpayers when buying a fleet of transport planes for the RAF, the spending watchdog has said.
The National Audit Office said the 14 Future Strategic Tanker Aircraft would be delivered more than five years late.
The planes would have to be fitted with suitable protection costing hundreds of millions of pounds before they could be used in Afghanistan, it added.
The MoD said the £10.5bn Brize Norton-based fleet would be value for money.
The dual-use Airbus A330-200 aircraft will be capable of transporting troops and carrying out air-to-air refuelling.
They were due to be delivered in 2006, but they will not be in service until the end of 2011.
Instead, the RAF is relying on ageing Tristars and VC10s to carry out such roles.
The report said the private finance initiative (PFI) deal faced years of further delays if the MoD were to decide the aircraft should be "retro-fitted" with flight deck armour and other protective equipment to enable them to operate in "high-threat environments".
The National Audit Office (NAO), Whitehall's spending watchdog, said the process could cost "several hundred million pounds" if it went ahead.
The NAO said that when the MoD originally began work on the procurement programme, it was not envisaged they could be required to fly directly into conflict zones, and no funding was provided for protective equipment.
Instead it was decided the additional equipment would have to be "retro-fitted" once the RAF started taking delivery of the new aircraft in 2011.
In the meantime, it said, the MoD was spending £23.5m replacing flight management systems and cockpit displays on the Tristars so they could carry on operating.
Overall, the NAO said it was impossible to assess whether the PFI deal provided value for money to the taxpayer and it criticised the MoD for failing to carry out a "sound evaluation of alternative procurement routes".
Edward Leigh, the chairman of the Commons Public Accounts Committee, which oversees the work of the NAO, strongly criticised the use of PFI.
"By introducing a private finance element to the deal, the MoD managed to turn what should have been a relatively straightforward procurement into a bureaucratic nightmare," he said.
An MoD spokesman said the Tristars currently in operation were "fitted with the highest possible standard" and the new planes' levels of protection would "match or exceed" these.
"The MoD is pleased the NAO has acknowledged that this FSTA [Future Strategic Tanker Aircraft] project has achieved all of its delivery milestones since the contract was signed," the spokesman added.
"We recognise that some aspects of the procurement in the early stages might have been improved but we are content that the UK has secured a good deal for the taxpayer and for the RAF."