Nasa astronauts were cleared to fly while drunk at least twice, a panel set up by the US space agency has found.
One incident related to a delayed shuttle flight
The astronauts were cleared to fly even though colleagues had raised safety concerns, the independent review found.
The panel also said it found evidence of heavy use of alcohol within the 12-hour pre-flight ban on drinking.
Nasa said it would launch a full-scale internal safety review and recommend unspecified corrective action if incidents were found to have occurred.
The independent review was begun to look at health issues affecting Nasa crew members after astronaut Lisa Nowak was arrested on kidnapping and assault charges.
Ms Nowak was arrested in February and stands accused of attacking her love rival, the girlfriend of a fellow astronaut.
The Astronaut Health Care System Review Committee's report does not mention her or anyone else by name. Neither does it detail mission specifics or dates.
The allegations about the astronauts first surfaced in the publication Aviation Week and Space Technology on Thursday.
On Friday, Nasa published the independent report which noted two episodes of particular concern.
"Two specific instances were described where astronauts had been so intoxicated prior to flight that flight surgeons and-or fellow astronauts raised concerns to local on-scene leadership regarding flight safety," the report said.
"However, the individuals were still permitted to fly."
It is unclear how many hours before the flight the drinking occurred and what quantities were consumed.
The chairman of the panel, US Air Force physician Col Richard Bachman, could add only a few details during a media conference.
One incident involved preparations for a shuttle mission that was eventually delayed, he said. The astronaut then wanted to fly on a T-38 trainer jet which is used by crew to move between Nasa's Houston centre in Texas and the Kennedy launch complex in Florida, but the Colonel gave no further information.
The second case involved a Russian Soyuz mission bound for the International Space Station (ISS).
The panel chairman said there was no way of knowing whether these were isolated incidents or the tip of the iceberg.
Col Bachman outlined a number of the committee's recommendations, including the institution of a formal written code of conduct, the creation of "enduring supervisory and mentoring relationships", and the empowerment of staff to bring forward concerns.
Nasa Deputy Administrator Shana Dale said the agency would institute an internal safety review to investigate the "allegations".
"We will act immediately on the more troubling aspects of this report with respect to alcohol use, and the anecdotal references to resistance by agency leadership to accepting advice or criticism about the fitness and readiness of individuals for spaceflight," she said.
"I must emphasise that this report does not provide specific information about alcohol-related incidents and the review committee has left it to Nasa to assess the scope of these alleged incidents."
However, Ms Dale welcomed the recommendation of a formal written code of conduct for astronauts, who she said represented some of America's most extraordinary and talented individuals.
"For almost the entire history of the astronaut core our experience has been that Nasa's astronauts conduct themselves with integrity, professionalism and a desire to bring honour to America and America's space programme," she added.
The space agency also said it would make explicit its policy that prohibits any drinking in the 12 hours before an astronaut flies.
The report comes at a tough time for the space agency, jolted by revelations on Thursday that a computer supplied by a contractor had been sabotaged.
The damage to wiring in a network box - which is to be taken to the International Space Station (ISS) - was intentional and obvious, the agency said.
However, it stressed that the equipment was not essential and that astronauts' lives had not been at risk. An investigation is under way.
Nasa's Associate Administrator for Space Operations, William Gerstenmaier, said the computer problem had been discovered earlier this month.
The computer is designed to collect and relay data from sensors which detect vibrations and forces on the space station's external trusses.
The equipment had been supplied by a sub-contractor, he added.
Mr Gerstenmaier said engineers would try to repair the hardware before 7 August, when the space shuttle Endeavour is due to fly to ISS, but that the mission would not be delayed.
The damage is believed to be the first act of sabotage of flight equipment Nasa has discovered.