The chart-topping singer, whose real name is Mark McLachlan, was distraught when he found out his dad John had died at the age of 79 at his home in Dalmuir, Clydebank.
Marti, who is due to return to Glasgow to perform in Blood Brothers at the King's Theatre, now has no surviving immediate family after losing his mum, Margaret, last year and his big brother, John, in 2011.
The 49-year-old is currently on tour with the award-winning musical, which opened at Cambridge Arts Theatre last night.
Despite being close to his former Clyde shipbuilder dad, Marti's spokesman, David Bloom, said he was "unavailable for comment" yesterday.
But close pals and regulars in the Cleddans Bar in Clydebank where John socialised for many years paid tribute to "a genuinely lovely man" and said his death on Monday was a "sad loss" to everyone who knew him
A friend said: "John was the nicest guy you could ever meet. He had no airs or graces. He was just a down to earth, funny and a truly great friend who will be sadly missed.
"He was so proud of his boys but never boasted about his son Mark being in showbiz.
"It didn't matter to him that his son was famous, he loved him for who he was and he was very close to both his boys.
"He had been fighting cancer for about a year and he had all the treatment, but there was nothing more doctors could do for him.
"I can't believe he's gone."
Like Marti, his brother John struggled with a drink problem for years.
John supported Marti in 1999 after he collapsed from an overdose in a London hotel and admitted to shocked fans that he was addicted to heroin.
The star fought back after a spell in rehab, and he visited his brother every day when John had treatment for his alcohol problem in the early 90s.
Marti has now carved out a successful new career starring in musicals but his dad didn't always support his idea of a career in showbiz.
When Marti was 17 and broke the news that he wanted to be a singer, his dad asked: "Are you on a hallucinogenic substance?"