A woman who died at Watford Tesco Extra yesterday was having breathing difficulties before she collapsed, according to the ambulance servce.
The 68-year-old woman took ill in the store shortly after 5pm.
It is understood she was with family members in the supermarket in Dalton Way when she became unwell.
Charlotte Parker, spokeswoman for East of England Ambulance Service, said: "We were called at 5.12pm for a woman who was having difficulty breathing.
"We sent an ambulance, rapid response vehicle and Hertfordshire Air Ambulance to the scene.
"The ambulance was on scene at 5.20pm and the helicopter landed at 5.34pm.
"Medics provided advanced life support.
"The woman went into cardiac arrest and was sadly declared dead at the scene."
Police closed the road leading into the Tesco to enable the air ambulance to land in the store car park.
A spokesman for Tesco has offered his condolences to the woman’s family following the tragedy.
Bill Glennie, store director for Tesco, said: "We were very sad to hear of the death of one of our customers at the Watford Extra store, and our thoughts go out to the family.
"Our colleagues will of course help the authorities in any way they can."