John Cowan, who would have turned 44 tomorrow, passed away earlier this summer after falling ill at the Hotel Kalofer in the country’s Sunny Beach resort.
His relatives say Jet2 have failed to provide answers and claim four other people were struck down with the disease at the same hotel.
Mr Cowan, a hotelier from Hamilton, died on 19 June as a result of legionella he contracted while on holiday.
He first became ill in Bulgaria and continued to suffer the effects of the virus until he died one week after returning home.
The causes of death formally listed on his death certificate are septic shock and legionella pneumonia.
Mr Cowan’s brother, Barry, said: “We are all devastated. Neither Jet2 nor the hotel have helped us at all - it’s like they’re just trying to hush it all up rather than help a bereaved family.”
The 41-year-old said Jet2 informed the family they had not tested the aircraft his brother had flown on.
He added: “We know of two other UK families affected by this who also stayed in the same hotel. Apparently there were also two hotel staff members who became ill.
“It was the hardest thing ever and I don’t think it’s something we’ll ever get over.
“We just want answers - to know why this happened, to know it will never happen to anyone else and to know why the tour operator and the hotel have done nothing to help.”
Mr Cowan said that insurers will not help the family with the funeral costs because John had died of an illness and not an accident. That outcome, he explained, left the family with “no option but to take legal action.”
Mark Gibson, a partner at Digby Brown Solicitors and head of its foreign and travel team, said: “We are currently investigating a claim that a man died as a result of
Legionnaires poisoning at a hotel in Bulgaria but as the case is at an early stage it would be inappropriate to comment further.”
Jet2 said there had only been three cases of people falling ill with the same virus.
A spokesman for the company said: “Sadly, we can confirm that two customers recently passed away shortly after returning to the UK from holiday in Bulgaria.
“We would like to offer our heartfelt condolences to their families at this very difficult time, and we have been in constant contact to offer all the assistance and support that we can.
“The health and safety of our customers is our absolute priority. In response, and to support our investigation, two independent specialists were instructed to take samples from the hotel and a UKAS accredited laboratory conducted testing of the water samples for any contamination.
“The results of all their tests were negative. We will continue to assist the local authorities in their investigations as required.”
He added: “Despite these results we have taken further precautionary measures, like other UK tour operators at the hotel. We have transferred all customers and all bookings for this summer to other hotels, and we have also put a stop on sales to the affected hotel for Summer 19 and Summer 20.”