Eton College was forced to apologise after an email was accidentally sent offering 400 pupils places at the school, instead of just nine.
The computer glitch meant students who had attended an interview at the college were told they had a conditional place as long as they passed the common entrance exam. However the email was only intended to be sent to nine pupils.
The boarding school in Berkshire has produced more prime ministers than any other school, and was attended by Princes William and Harry, as well as Boris Johnson.
It will now be holding an internal review, Sky reported, with a spokesperson saying: "This error was discovered within minutes and each family was immediately contacted to notify them that it should be disregarded and to apologise.
"We take this type of incident very seriously indeed and so a thorough investigation, overseen by the Head Master, Tony Little, and led by the Tutor for Admissions, is being carried out to find out exactly what went wrong and ensure it cannot happen again.
"Eton College offers its sincere apologies to those boys concerned and their families. We deeply regret the confusion and upset this must have caused.
"We understand the problem was caused by a computer error. Our review will seek to discover what went wrong and put in place appropriate solutions, but we would not wish to pre-empt its findings at this stage.
"We have contacted and apologised to all the families concerned."