A Virgin Atlantic employee has resigned amid allegations she passed on the flight details of eight celebrities to a paparazzi agency.
The airline is looking into claims the woman handed the information to London-based Big Pictures and said it would "deeply regret" any concern caused to the passengers involved.
The Guardian newspaper reported that stars whose flight details were allegedly passed are Chelsea footballer Ashley Cole; his former wife and singer Cheryl Cole; Tottenham footballer Jermain Defoe; actresses Sienna Miller, Scarlett Johansson and Gwyneth Paltrow; and singers Robbie Williams and Nicole Scherzinger.
A Virgin Atlantic spokeswoman said: "The allegations that have been raised are extremely serious and we have launched an immediate investigation.
"The security of customer information is our highest priority and we have robust processes in place to ensure that passenger information is protected.
"The incident that has been alleged concerns eight customers' flights booked in 2010 and we are in contact with all of those people.
"It is too early to draw conclusions on this matter but of course we would deeply regret any concern that these allegations may cause the individuals involved."
A source at the airline Sir Richard Branson launched in 1984 said it was scanning through email archives and investigating who could access its booking system.
The accused employee, who held a relatively junior position, denies the allegations, it is understood.
A spokeswoman for Defoe said: "This is a serious breach of privacy. We'll be consulting with our lawyers and investigating the matter further."
A spokeswoman for Miller, who obtained an injunction against the paparazzi in 2008, said she had no comment to make.
The actress has previously given evidence to the Leveson Inquiry into press standards, describing the intrusion she said she suffered at the hands of photographers.
She told the hearing in November: "I would often find myself - I was 21 - at midnight, running down a dark street on my own with 10 big men chasing me.
"And the fact they had cameras meant it was legal. Take away the cameras and what have you got? A pack of men chasing a woman, and obviously that's a very intimidating situation to be in."
Big Pictures, which launched in 1992, claims to be the world's largest celebrity picture agency, supplying pictures to 40 countries.