A United States Navy report into the deaths of two submariners off the Devon coast says a harbour pilot gave the crew "poor advice" about the weather.
The sailors died after being swept off the USS Minneapolis-St Paul in strong winds in Plymouth Sound in December.
The report also claimed the Royal Navy had not shared details of a similar incident a few months earlier.
A spokesman for the union Prospect, which represents harbour pilots, said the criticism was "unbelievable".
A Royal Navy spokesman said they welcomed the report and were working with the US Navy.
'Dangerous seas'
Four sailors were swept from the deck after the submarine left Plymouth harbour on 29 December in rough seas.
They were taken to Derriford Hospital, where two - Senior Chief Petty Officer Thomas Higgins, 45, of Paducah, Kentucky, and Petty Officer 2nd Class Michael Holtz, 30, of Lakewood, Ohio - were pronounced dead.
Following the accident, the submarine commander Edwin Ruff was removed from his post and reassigned to a position on shore.
The US Fleet Forces Command investigation found a Plymouth harbour pilot had not shared concerns about "potentially dangerous sea states" in the area with the American crew.
But it concluded that the submarine crew should have been able to anticipate any rough seas and had incorrectly analysed available weather forecast information.
A spokesperson for the Ministry of Defence, which oversees harbour pilot operations for the Devonport naval base, would not comment on the actions of the pilot.
But she said a team from Devonport was due to discuss the incident at the home port of USS Minneapolis in Virginia.
Rescue teams commended
In a similar incident in the summer of 2006, Royal Navy sailors were injured as their submarine was transiting in rough seas past the Plymouth breakwater.
The US Navy report said had the ship's crew known about it, they would have considered the effects of the weather beyond the breakwater.
The report also praised the work of Plymouth rescue teams and recommended US Navy commendations for their efforts.
The nuclear-powered USS Minneapolis-St Paul is based at Norfolk, Virginia.
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