he lamb in a sealed package was bought by Kathryn Chambers and partner Paul Burns in the Asda at Main Street in Bangor.
Bluebottles emerge from eggs which hatch into maggots before turning into flies in a process that takes several days.
The Asda-branded lamb was from Northern Ireland, according to the packaging.
Mr Burns said: “Entomology and logic dictates that a blue bottle had laid its eggs on this exposed meat in an ambient temperature at the packing plant and the pupation process would follow both during and after sealing in the packing at the factory plant.”
A former firefighter with more than three decades’ experience, he has had direct experience of forensically studying scenes, including establishing timelines based on the life cycles of insects.
Asda said: “We are very sorry that Mr Burns purchased a product from one of our stores which fell far below our usual standards.
“This was the only complaint that we have received about this and we have strict systems in place to prevent these kinds of incidences occurring, so we are investigating what could have happened with our supplier.’’
Ards and North Down Borough Council said it had “received a complaint alleging flies in packaged chilled meat purchased from a local supermarket”.
“Officers are currently investigating this in conjunction with DAERA colleagues. As this is an ongoing investigation we are unable to give further details at this stage,” it added.
In a letter from Asda’s quality control department, Mr Burns was told the company was “sorry there were several flies in the packet of lamb”.
“I can understand how distressing and unpleasant this must have been for you,” the company said. “This certainly is not the condition we want any of our products to arrive in.
“We use strict quality control systems in all of our factories to prevent this kind of thing of happening.
“I am disappointed our standards slipped on this occasion.”
The company promised it could track down the batch, look into what happened and “make sure we get things right in the future”.
Mr Burns described how Ms Chambers bought the stewing lamb on the Saturday, placed the container in a storeroom fridge, then took it out a number of days later while preparing dinner.
It was at that point she noticed the five live bluebottles inside the sealed packaging.
The couple took photographs and video footage.
“I examined the package and its contents and noted that the package was sealed because, self-evidently, the bluebottles remained inside the package;” he said.
“I can categorically state there were no perforations in the sealing plastic membrane where they could either enter the package or leave.”
He brought the meat back to the store, where it was examined by several employees.
“I supplied them with my contact points and was taken to customer services, where I handed over the package to a young man, who took it from me, smelled it, inspected its live contents and then reimbursed my card with a £10 gratuity,” he added.
He later received another £10 voucher from Asda.
He followed up with messages to Asda and to Ards and North Down Borough Council’s environmental department.
Mr Burns added: “This is not about any compensation, this is such a serious health issue it needs to be investigated and highlighted.”