A cigarette discarded in guttering caused a blaze that destroyed the roof and upper floor of Yummy Pub Co's the Somers Town Coffee House, in Euston, on 22 June.
Fire crews arrived at the pub shortly after 8pm and managed to save the ground floor, although the roof and second floor suffered extensive damage. The areas included staff accommodation and a chef academy, where the pub group worked with people from a neighbouring homeless charity.
The company, led by Anthony Pender and Tim Foster, said a forensic investigation had found that the discarded cigarette had set light to guttering on the second floor of the pub.
Foster said: "We now understand the fire began at the back of the pub on the upper floors. It's terrifying to know that a business you have built for the past 10 years is put at risk due to somebody's careless attitude. What's more unforgiving is that we have since found fresh cigarette butts in the area where the fire began, meaning somebody is still discarding fag ends after the fire! Police have been informed and we're installing cameras to that area of the pub.
Foster, who has been shortlisted for the Education and Training Award at the Cateys 2018, praised his staff for evacuating 350 people from the pub within three minutes of the fire's discovery.
When the pub reopens next week, Yummy Pub Co will give a free pint to anyone wearing a hard hat, with a party to thank London Fire Brigade also planned for the coming weeks.
London Fire Brigade issued a smoking safety reminder following the blaze.
Station manager Steve Chesson said: "This incident shows how important it is to ensure your cigarette is completely out when you've finished smoking it.
"If it's not, you risk causing a fire which could not only destroy property, but could also cost you your life."