Enterprise Inns has today been fined £300,000 after a licensee died from carbon monoxide poisoning, and tenants at another 474 pubs were put at risk.
Paul Lee was found unconscious by a cleaner at the Aintree Hotel in Bootle, Liverpool, on November 12 2007.
He had turned on a gas fire in his living room 10 hours earlier before falling asleep.
The 41-year-old suffered a heart attack due to lack of oxygen on the way to the hospital and died the following morning without regaining consciousness. He had worked as the tenant at the pub for less than a month.
Enterprise Inns was prosecuted after a Health and Safety Executive (HSE) investigation found that the fire may not have been serviced since 1979 and the chimney was completely blocked.
The company, which owns approximately 7,700 pubs admitted breaching Section 3(1) of the Health and Safety at Work etc Act 1974.
Liverpool Crown Court heard that Enterprise Inns should have ensured that gas safety inspections were carried out at 868 of its pubs at least every 12 months, but that only 394 had valid certificates. The gas heater which caused Mr Lee's death should have been checked before he took over the tenancy.
Enterprise Inns also received a written warning from HSE in 2001, following a fire at one of its properties in Birmingham, which highlighted a systematic failure to implement annual gas safety checks.
Sharon Lee, Mr Lee's sister, said: "Since Paul's death nearly three years ago, there is still anger and disbelief amongst his family and friends that it was entirely preventable.
"Paul's death will very possibly save the lives of others in the future, but it should not have taken the loss of his life to highlight the wider failings of Enterprise Inns.
"We are fully aware that Enterprise Inns is now compliant with gas safety legislation, but companies must not put other people's lives at risk by allowing similar lapses to occur in the future."
Iain Evans, the investigating inspector at HSE, said: "It is shocking that a major pub chain failed to ensure regular gas safety checks were carried out at more than 400 of its properties. As a result, one man has been killed and hundreds of other lives have been put at risk.
"Tests we carried out on the gas fire at the Aintree Hotel showed that the workplace limit for exposure to carbon monoxide would have been exceeded within five minutes of it being turned on, and would have reached a level known to be fatal within an hour.
"The chimney from the fire was completely blocked so there was nowhere for the carbon monoxide to escape. Instead, it gradually built up in the room and starved Mr Lee's organs of oxygen until he was left unconscious.
"What makes this case so tragic is that Mr Lee's life could have been saved if Enterprise Inns had continued to obey the written warning it received about gas safety six years earlier, instead of falling back into old habits."
Enterprise Inns was ordered to pay £19,000 towards the cost of the prosecution in addition to the fine at Liverpool Crown Court today.
A spokesman for Enterprise Inns said: "We would again like to express our sincere apologies to the family and friends of Mr Lee for his tragic death. There were a number of factors which contributed to this incident; both in systems, processes and human failings.
"Since 2007, working closely with the HSE, Enterprise has done everything possible to ensure nothing like this can ever happen again."
Landlords can check whether a gas engineer is registered by visiting: www.gassaferegister.co.uk or calling 0800 408 5500.