A suspicious item discovered at Old Trafford on Sunday was a training device, police have confirmed.
The match between Manchester United and Bournemouth was called off after the device was found shortly before kick-off. Bomb disposal officers were called and carried out a controlled explosion.
Police said the item had "accidentally" been left by a private company after a training exercise on Wednesday.
The Premier League match has been rearranged for Tuesday at 20:00 BST.
Greater Manchester's Mayor and Police and Crime Commissioner Tony Lloyd called the situation "unacceptable" and wants an "urgent" inquiry to discover "how this happened, why it happened and who will be held accountable".
He added: "This fiasco caused massive inconvenience to supporters who had come from far and wide to watch the match, wasted the time of huge numbers of police officers and the army's bomb squad, and unnecessarily put people in danger, as evacuating tens of thousands of people from a football stadium is not without risk."
United will be refunding all tickets and allowing ticket holders from both clubs in for free on Tuesday night. Manchester United season ticket holders will also be given a credit.
It is estimated this gesture will cost United around £3m.
Bournemouth expect to make an announcement on arrangements for their travelling fans on Monday.
A Greater Manchester Police (GMP) statement read: "Shortly before today's planned football fixture, staff from the Manchester United ground alerted police to a suspicious item that had been found in the toilets within the North West Quadrant, between the Sir Alex Ferguson stand and the Stretford End.
"Police quickly attended and explosive experts were called in to assess the item, which has been described as an incredibly realistic-looking explosive device."
The Sir Alex Ferguson Stand and the Stretford End were evacuated and sniffer dogs brought in.
Kick-off was delayed initially and shortly afterwards the match was abandoned on police advice.
A bomb disposal team carried out the explosion at about 16:30 BST.
Assistant chief constable John O'Hare said: "I am grateful to the Manchester United and Bournemouth supporters for their support and assistance today.
"Following today's controlled explosion, we have since found out that the item was a training device which had accidentally been left by a private company following a training exercise involving explosive search dogs.
"While this item did not turn out to be a viable explosive, on appearance this device was as real as could be, and the decision to evacuate the stadium was the right thing to do, until we could be sure that people were not at risk."
Manchester United's executive vice-chairman Ed Woodward said: "The club takes security very seriously and staff are regularly trained with the police and emergency services to identify and deal with these incidents.
"We will investigate the incident to inform future actions and decisions."