The Sunday Times brought to light the fact that the Civil Aviation Authority measures departure times from when an aircraft leaves the terminal buildings, and not when it takes off. However, the British Airways T5 ad said that 90% of flights were "taking off within 15 minutes of the scheduled time", which it had said was an industry standard.
The copy on the BA website now reads: "Yesterday at T5 90% of flights left on time."
The Sunday Times reported yesterday that a source at BA blamed the T5 problem on a "mix up" between BBH and BA's marketing staff.
the source told the newspaper: "We are so annoyed because we knew we had to be spot on with the wording but someone at the agency changed it. We should have picked it up."
The airline is investing £6m in the T5 ad campaign, in a bid to change the minds of the public following the debacle of the opening of Terminal 5, which saw flights cancelled and delayed, passengers queuing for hours to check in, and luggage lost.
The "Terminal 5 is working" campaign will keep passengers updated as the punctuality of flight arrivals and departures, as well as average check-in times, as reported by passengers using the terminal.
A spokeswoman for BA, said: "This was a genuine human error and was an innocent oversight.
"The headline claim of 90% of flights leaving from the terminal on time within the industry standard was correct, however the use of the phrase "take-off" may have been unintentionally confusing and for that we apologise.
"All 90 airlines which use Heathrow have to queue for take-off for around 20 to 25 minutes most days. This process is not controlled by the airlines.
"Overall we are pleased that Terminal 5 continues to work well and our customers are enjoying the best levels of service at Heathrow for many years."
Virgin Atlantic, the bitter rival service, is understood to be preparing to complain about the campaign to the Advertising Standards Authority.
Separately, Virgin Atlantic is to undertake a £3m campaign in a bid to scupper British Airways' plan to set up a joint venture with American Airlines. The advertising and lobbying push will argue that the plan is dangerous for consumers.