The suspect device which led to the abandonment of Sunday's game at Old Trafford could not have been detected by a routine sniffer dog search, Manchester United have said.
The fake bomb, accidentally left by a private security firm, had also been reported as recovered following a training exercise.
The points were two made by executive vice-chairman Ed Woodward in a fresh statement into the security alert which led to the 75,000-capacity stadium being evacuated and a controlled explosion carried out.
Woodward said: "The safety of the fans is our number one aim at every event we host at Old Trafford. Overall, I'm proud of how our staff responded.
"On the discovery of a suspect package, the police and the club worked quickly and closely to identify the threat, make people safe and evacuate the ground calmly and efficiently.
"Fans of both clubs behaved impeccably and the evacuation - the first of its type in the UK - was a complete success.
"Following investigation, the device proved to have been left in error following the training of dog handlers by a sub contractor.
"We have worked very closely with the police and counter terrorism specialists for many years now and enjoy their support on a daily basis." - Ed Woodward
"The contractor had signed the device as having been recovered along with the 13 other devices at the end of the exercise. That device could not have been detected by sniffer dogs on the routine matchday search of the 100 Club, as it contained no explosives and was used in an exercise training handlers not dogs."
The dummy bomb was initially described as an "incredibly lifelike explosive device" after it was discovered in toilets at the stadium shortly before the scheduled 3.0pm kick-off of United's clash with Bournemouth.
An evacuation began at 2.40pm with the item being destroyed by bomb disposal experts from the army at 4.30pm. It was later revealed the device had been used in a training exercise last week.
Woodward promised the matter would be investigated but has praised those involved in what proved a highly efficient evacuation.
He said: "Once a live situation was identified, the club and police had no option but to treat the matter as a potential terror threat; we could not have assumed it was a training exercise error. Presented with the same situation in the future, we would take the same action.
"We have worked very closely with the police and counter terrorism specialists for many years now and enjoy their support on a daily basis.
"We are conducting a detailed evaluation with the help of the police and will share our findings across the rest of the game. Valuable lessons will have been learned from yesterday's events and it is important that those are shared with other stadium operators to ensure that the safety of the public remains the first duty of us all."
The security company at the heart of the incident has not been officially named but is understood to be Security Search Management & Solutions Ltd.
The company's managing director Chris Reid, when approached for comment, said: "It is a very unfortunate situation and the consequences obviously were that a lot of people were inconvenienced. Fortunately everybody was fine which is the most important thing."
Speaking briefly outside his home in Biggin Hill on Monday morning, Mr Reid said he would like to say more about the "security situation" but was waiting on United to contact him.
Woodward's update came after a strong call from Greater Manchester's police and crime commissioner for answers.
Tony Lloyd, who is also the mayor of Greater Manchester, said: "I think United have to come up front with all this because in the end it's their reputation, but it's also public safety and both those two really do matter.
"Fiasco is the right word. It was shambolic. Of course United are a huge organisation. It wasn't the fact they're the world's richest club - that they are - it was the fact that the security had missed something that in the end ought to have been found."
