Mr Letts, the sales director for the FLB Group, was found lying in the driveway of his home in Duns on Sunday evening after returning from a run.
A former tank commander in the army, he was described as an active sportsman who played polo, tennis, cricket and was also a skier.
He was married to Rosie with whom he has three children, Emily, 14, Harry, 13, and Louisa,11.
“It has hit us like a sledgehammer. He was a very fit guy,” said Gordon Presly, chief executive of the company who had worked with Mr Letts for 18 years.
The pair were involved in two management buyouts in 2000 and 2006. In 2001 they acquired Filofax which became famous in the pre-smartphone 1980s as the must-have personal organiser.
Mr Presly said in a statement: “It is with an unfathomable level of sorrow and sadness that I have to inform you of the sudden death of Charles Letts.
“This whole situation is almost too tragic and heartbreaking to try and put into words.
Charles was an immensely popular and likeable individual with many close friends and acquaintances. He will be hugely missed by his family, his friends and his work related associates.
“Our deepest sympathy and sincere condolences go to Charles’ family who, as you would expect, are utterly devastated and numbed by his sudden and unexpected passing.
“I would also ask that their privacy is respected at this hugely sensitive time.”
Mr Letts joined the family firm in 1990 at the age of 25.
It is understood that there was a delay in releasing details of his death until one of his children, who was overseas, had been told.
There was no confirmation of the cause of death and no funeral arrangements were known, but Mr Presly said he was discovered in his running gear and it appeared he had suffered “some form of seizure”.