Critics said the chickens would have suffered terribly before dying.
Workers at Moy Park farm in Newton on Trent told The Lincolnite: “We tried to do everything but there was nothing more we could do. The freak weather has done this to them. Please don’t turn this into anything bad.
“It has been really tough carting these animals out of the farm over the past couple of days. Animal activists don’t think that we care about them, but we really do.”
Northern Ireland-based Moy Park, which says it is one of the UK’s top 15 food companies, supplies supermarkets including Tesco, Sainsbury’s and Ocado.
Moy Park farms are endorsed by the Red Tractor scheme, which says its members provide food that is “farmed with care” and “responsibly produced”.
Farm staff were seen piling up the corpses outside the sheds on Friday.
Another critic tweeted: “No excuse. Technology exists to cool buildings like this to keep the inside at a comfortable temperature.
“Even on normal summer days those tin boxes must be uncomfortable. Yes it would cost money, but I’m guessing consumers would sooner pay more than see stories like this.”
Compassion in World Farming says most chickens reared for meat are confined in industrial, barren sheds where birds cannot avoid the heat as they would in natural conditions.
“It can get very hot inside the sheds, especially in summer. If the ventilation system fails, thousands of birds can die of heat stress,” the organisation says.
A spokesperson for Red Tractor said: “Animal welfare is our top priority and we require all certified poultry farms to do all they can to protect their birds. Record temperatures last week posed significant challenges for livestock, pets and humans alike.
“We will continue to work with the poultry industry to review how to minimise the impact of extreme weather conditions on their farms.”
A spokesperson for Moy Park, which processes more than 280 million birds a year, said: “We are working closely with our farming partners to monitor the situation and have implemented procedures to help protect our birds against the extreme heat.” It was not immediately clear whether that was after Thursday’s disaster.
He could not say how many birds died of the heat, nor whether any survived.
A spokeswoman for the British Poultry Council said she was not aware of any previous cases of mass deaths caused by extreme weather.
Farmers worked to Red Tractor best practice guidelines, including providing extra ventilation and temperature-controlled systems, and being more vigilant, she said.
Tesco and Ocado have been asked to comment.
A Sainsbury’s spokeswoman said: “Animal welfare is extremely important and we’re in close contact with our suppliers about last week’s record temperatures.”