The Love Will Tear Us Apart hitmakers came to an end in 1980 following Curtis' suicide that year, and the remaining members went on to achieve huge global success as New Order.
Curtis had been plunged into despair by relationship issues and his epilepsy diagnosis, and Sumner now realises the band was "in denial" about the extent of the singer's problems and believes JOY DIVISION could not have carried on even if Curtis had lived.
Sumner tells Mojo magazine, "He had epilepsy. Could he carry on with the band with epilepsy as bad as he had? He couldn't really. We were all in denial about it, but when you think back, the writing was on the wall. He couldn't have carried on. He was too ill. Maybe he felt he was letting us all down. He wanted out, but he couldn't bear letting us down because we'd all fought so hard to make it a success."