The Foreign Office said it was only recently discovered that records of flights passing through an overseas territory used by the US for extraordinary rendition had been lost to "water damage".
A spokeswoman said:
The British Indian Ocean Territory administration is responsible for records of flights on Diego Garcia and they are investigating the full extent of the damage, how many records and what information is affected.
The damage was only recently discovered so it is unknown how exactly or when it occurred.
More top news
Rain in the north tonight, cloudy and humid in the south
The 26-year-old had just given birth to her third child and was being transferred to a different floor when the tragedy happened.
Millions have stopped to watch the sun blotted out for a total solar eclipse across a swathe of North America.