Steve Barker, from Chorley, Lancashire, went to the doctors over Christmas for a different matter and mentioned a "tiny" lump on his chest.
After doctors referred him for a scan, he found out the lump was the tip of a 2.8lbs (1.3kg) bone tumour.
Surgeons at Blackpool Victoria Hospital who removed the tumour said it was the biggest they had even seen.
Mr Barker, 57, said: "It was a shock really, I quite rapidly tried to get my head around it.
"I was amazed that behind the lump, inside my chest, was this large tumour pressing against my lungs, my heart, my back."
Mr Manoj Purohit, who led the operation, said the tumour was as "big as a football" and the largest tumour he had ever come across.
"The tumour was so large that it had pushed the heart from the front centre of his chest to the back left and also had started compressing the lungs making it difficult for Mr Barker to breathe.
"If it had been left unnoticed much longer it would have kept on growing and would have proved fatal."
The operation required removing the breast bone and five of Mr Barker's ribs. They were then replaced using prosthetics.
Mr Barker recovered quickly leaving hospital just seven days after the three-hour operation.
Mr Barker added: "I was just glad to get the thing out of my body to be honest. The recovery has taken a while, but I feel far better now than I did when I was in hospital.
"The message is you mustn't feel embarrassed about going to speak to your doctor - ninety-nine times out of a hundred, it will be absolutely nothing.
"But the sooner they find the thing, the better it will be for you and the doctors."
A man who had a giant tumour "as big as a football" removed has said he is lucky to be alive.