A $15bn (£7.6bn) contract for US Air Force rescue helicopters won by Boeing should be reopened, a key US government watchdog body says.
The order is for HH-47 helicopters, which are a variant of the Chinook
Lockheed Martin and Sikorsky had complained that Air Force evaluation of their bids was not in line with the original tender.
The Government Accountability Office (GAO) says the Air Force should ask the three firms for revised proposals.
Boeing shares fell 1.5% in New York on Monday, closing down $1.35 at $88.93.
According to the GAO, the US Congress's main spending watchdog, the fresh bids will need to be reviewed by the Air Force.
If they show that Boeing's proposal for the 141 helicopters no longer represents the best value for the government, the contract should be terminated, the GAO says.
The recommendation is not binding, but the Air Force must respond within 60 days.