The RAF's most advanced fighter jets have been grounded owing to shortages of spare parts, MPs have found.
The Commons public accounts committee said five Typhoon pilots were grounded last year because they had been unable to put in enough training flight hours.
The jets have been used in Libya, and were deployed for the first time in a ground attack this week near Misrata.
The Ministry of Defence insists the problems identified by the committee have since been addressed.
Although Typhoons - which are made by BAE systems in Warton, Lancashire - have already been carrying out air defence missions, they have only recently been equipped for a ground attack role and were deployed in a bombing mission for the first time this week in Libya.
In their report, the MPs warned that only eight of the RAF's 48 Typhoon pilots were qualified to carry out ground attacks last year.
But Air Vice Marshal Phil Osborn said that there were now "sufficient Typhoon aircraft and pilots to undertake the task in Libya".
The RAF was undertaking its mission in a "proportionate, disciplined, reliable way," he added.
The committee claimed that the RAF was having to cannibalise aircraft for spare parts in order to keep the maximum number of Typhoons in the air on any given day.
It added that the Ministry of Defence (MoD) had warned the problems were likely to continue until 2015 when it expects the supply of spares finally to have reached a "steady state".
The committee said that the MoD relied on a "small group of key industrial suppliers who have the technical and design capability to build, upgrade and support" the jets.
"Problems with the availability of spare parts have meant that Typhoons are not flying as many hours as the department requires," it added.
"The Typhoon supply chain is complex and stretches across Europe. However, the department admitted that it had not been managed well enough or delivered all the required parts when needed."
The committee noted that the MoD was now buying 30% fewer Typhoons than it had originally planned.
But the cost of the project had risen by an estimated £3.5bn, meaning that the expected cost of each aircraft had increased by 75%.
The overall cost of the programme is now estimated at £20.2bn, with the cost per plane rising from £72m to £126m.
The committee complained that the MoD had been unable to offer a "coherent explanation" for a decision in 2004 to equip the early Typhoons for ground attack operations at a cost of £119m, only to switch them back to an air defence role in 2009, a year after the upgrade was finally ready.
"The history of the Typhoon fighter aircraft represents yet another example of over-optimism, bad planning and an unacceptably high bill for the taxpayer," committee chair and Labour MP Margaret Hodge argued.
"This pattern of decision-making is more about balancing the books in the short-term rather than ensuring value for money over time."
Defence Secretary Liam Fox said the committee had recognised that the Ministry of Defence and the aerospace industry had been working to "resolve spares issues".
"Performance targets are now being met", he said.
The Minister for Defence Equipment, Peter Luff, said it was normal practice to take spares from smaller aircraft to maintain a large fleet.
"This was an oustanding aircraft, doing a great job for the UK and I don't think we should get too concerned for this particular aspect of the procurement," he said.
"Perhaps the biggest concern (is) some of the ways the decision was taken earlier in the process, which led to some of the cost overruns the committee rightly emphasised."