It is also the fourth BA systems failure in a month, with similar problems on June 19, July 7 and again on July 13.
One BA source told The Sun that the new IT system could cause a 'summer of holiday chaos'.
The source added: 'The system isn't robust enough for an airport like Heathrow. It's a nightmare'.
Passengers arriving at Heathrow yesterday are confronted with new signs, that say: 'We are currently upgrading our check-in system and with more than 700 flights a day, it's a big change for us. We would like to apologise in advance if it takes longer than usual to drop your bag today.'
Phillip Schofield was among several people who poured scorn on the message, saying: 'What are you upgrading too(sic)? Windows 2.1?!!!'
Other passengers said it was 'mayhem' and said the flashy Terminal 5 'is starting to resemble a 3rd World country domestic airport.'