The victims, a man and a woman, had been booked to stage an evening display at a luxury property in Ecclerigg, Cumbria, next to Lake Windermere.
They had joined about 70 guests who had gathered to mark the wedding of local insurance firm boss John Simpson and his partner Nicole Rothwell who married earlier in the week.
Witnesses and neighbours spoke of an "almighty bang" on Saturday followed by a series of "smaller bangs and pops", with one comparing the initial sound to that of a wartime bombing. A shed containing the fireworks erupted into flames and spread to another building and a parked car nearby.
Witnesses at Brockhole Lake District Visitor Centre said the noise of fireworks went on for about 20 minutes before firefighters arrived on and doused the blaze.
The Tree Top Trek adventure centre at Brockhole was evacuated as a precaution.
A man who attended the celebration, who did not wish to be named, said: "There was this almighty bang and then the fireworks started going off. We were wondering why it was being held in the bright daylight.
"The shed was somewhere behind the marquee so I couldn't see the flames but there was debris all around and I could see a large plume of smoke. All the guests were running away. It was terrible."