The probe has been launched following the incident at Asda’s North East Clothing warehouse in Washington.
Union chiefs have joined staff in raising concerns about the actions of the employee, with the firm confirming ‘strict procedures’ were not followed.
The box, which the Echo understands was bearing the words “This is a land mine from IS” and Arabic writing was delivered on a truck.
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One angry worker, who asked not to be named, said: “We do get a lot of stock from the Middle East and we’re in times of heightened terrorism.
“Asda is also part of Walmart which has a market globally, it’s a huge company.
“A colleague found it and went to speak to someone else, and you would think it would be all about health and safety, but they came, shook the box and opened it, when it should have been escalated.
“The police came and checked out that it was nothing, but it still put the depot at risk.
“It could have been catastrophic, there are up to 200 people on the site at about that time, so they put everybody at risk and the surrounding area as well.
“There’s a lot of anger over it and a lot of people have kids, which brings it all home.
“They’ve sent round a message saying its a learning curve.
“I think what’s happened is disgraceful.
“The rules have been flouted, there’s a bomb alert procedure, and that’s now been reiterated out.”
Michael Hopper, regional organiser for the GMB, says it is supporting the company as it also looks into how the message came to be written on the box.
He said: “We have safety concerns because if there had been explosives, there would have been a real risk.
“We are working with Asda as we relay the fears of staff members and make sure that policy is followed in future.”
A spokeswoman for Asda said: “We have strict procedures in place in our depots and we’re investigating why these weren’t followed on this occasion.”
A Northumbria Police spokeswoman said: “On October 3 police were called to the distribution centre in Alston Road in Washington over concerns about a package. “Officers attended and nothing of concern was identified.”