Tour firms’ established practice of leasing RAF aircraft to fly holidaymakers has come under the consumer media spotlight.
Three Airbus A330s are being used by Thomas Cook Airlines and Jet2.
The military aircraft are part of a fleet costing taxpayers £10.5 billion because the RAF has no use for them, the Mail on Sunday reported.
The aircraft are supposed to transport troops and refuel bombers in mid-air.
Although the Ministry of Defence is paying millions of pounds a year for the aircraft, the travel companies are using them to fly tourists to Tenerife, New York and Florida.
They are among a fleet of 14 Voyager A330s acquired by the MoD under a leasing agreement that costs about £400 million a year.
The deal with a consortium of manufacturers called AirTanker allows the commercial group to make millions more by renting up to four of the aircraft to tour operators when the RAF is not using them.
The agreement was signed in March 2008 when Gordon Brown was prime minister, Alistair Darling was chancellor and Des Browne was defence secretary.
They agreed to lease the Voyagers from AirTanker, a consortium that included Airbus and Rolls-Royce, under a private finance initiative arrangement.
The costs are paid back over 27 years, with repayments expected to peak at £593 million in 2034.
While AirTanker can lease the aircraft out, the MoD continues to pay for the right to take them back if required for military operations, the newspaper reported.
The deal was criticised and the National Audit Office said it was too expensive, uncompetitive and overly complicated, and that the MoD lacked the expertise to manage the project.
The MoD said: “Voyagers are playing a key role in protecting British airspace and in the fight against Islamic State, and while our core fleet is doing the job we need it to, we can call in the extra aircraft at short notice if we need them.
“When we’re not using them it makes sense to have them performing other roles, as that reduces our costs and means they can be ready for operations quicker than if they were in storage.”