The suspect package which led to Manchester United's Premier League game against Bournemouth being abandoned was a dummy device left over from a training exercise.
Greater Manchester Police (GMP) made the extraordinary announcement after a controlled explosion was carried out on the item at Old Trafford.
Police and security staff were forced to evacuate the stadium shortly before the scheduled 3pm kick-off on Sunday and the game was called off soon afterwards.
GMP said in a statement: "A suspicious item found at Old Trafford football stadium, which required a controlled explosion to be carried out, has now been identified as a training device."
GMP assistant chief constable John O'Hare said: "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.
"Whilst 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.
"Everyone remained calm, followed instructions, and worked with officers and stewards to ensure that a safe evacuation was quickly completed. Those present today were a credit to the football family and their actions should be recognised.
"I would also like to thank all those involved in the operation today for such a professional response, which includes police officers, stewards, MUFC staff, media representatives and commentators and the bomb disposal team."
After the stadium was given the all-clear, it was announced the game had been rescheduled for Tuesday night.
United's executive vice-chairman Ed Woodward revealed the incident would be fully investigated by the club.
Woodward said: "The safety of fans is always our highest priority.
"I'd like to thank the support from the police which was first class and the impeccable response from fans of both teams.
"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."