The US Air Force could waste nearly $9 billion on purchasing, operating and maintaining unmanned air vehicles it does not need, the Pentagon Inspector General (IG) has found.

The IG had looked into whether the air force had justified purchasing a total 401 General Atomics Aeronautical Systems MQ-9 Reapers, the larger, longer-range cousin of the MQ-1 Predator. It found that the air force had not justified the $76.8 billion price tag.

USAF Air Combat Command (ACC) officials failed to follow proper procedures to obtain approval for an increase in procurement, the report finds. The air force also did not “conduct and maintain consistent, complete, and verifiable analyses for determining the necessary aircraft quantity.”

“As a result, the Air Force risks spending approximately $8.8 billion to purchase, operate, and maintain 46 MQ-9 aircraft it may not need,” the IG finds.

The Air Force requested funds to procure 83 more Reapers its fiscal year 2015 President's Budget Request. The aircraft would be purchased in batches each year through 2019 at a total cost of $6.47 billion, according to budget documents. The per-unit cost of an MQ-9 in 2015 dollars is $20 million.

The IG recommends that the director of plans, programmes and requirements at ACC headquarters perform a rigorous review of how many Reapers are needed to perform specific missions, including training, test, Air National Guard needs, backup and “attrition reserve.”

That analysis will be submitted to the air force requirements oversight council and the joint requirements oversight council for approval of cost and quantity before any further purchases are finalised. Corrective action should be taken against MQ-9 requirements officials to “ensure analysis is conducted and maintained”, the IG suggests.

Only a summary report is available to the public without filing a Freedom of Information Act request. The summary said air force officials have accepted the IG’s recommendations.

Air Force officials fired back in a press release on 2 October, saying the IG’s data was stale.

"The data in the report is a year old," said Lt Gen Robert Otto, air force deputy chief of staff for intelligence, surveillance and reconnaissance. "The Air Force is only buying 346 aircraft, 55 fewer than the 401 figure contained in the report and we've only received about half that number. The projected total Reaper fleet of 346 is far less the service had originally estimated prior to sequestration."

Officials quoted in the statement said the MQ-9 has contributed heavily to combat operations in Iraq, Afghanistan and elsewhere. The aircraft is being used to fight Islamic State militants in Iraq and Syria.