The pair became close friends after starring together in the original Star Trek television series and films as Captain Kirk and Mr. Spock respectively.
However, William took to Twitter on Saturday to reveal his disappointment that he wouldn’t be able to say a proper goodbye to Leonard, who died on Friday aged 83.
“I am currently in FL as I agreed to appear at the Red Cross Ball tonight. Leonard’s funeral is tomorrow. I can’t make it back in time,” William tweeted.
“I feel really awful. Here I am doing charity work and one of my dearest friends is being buried.”
William continued to add that he is hoping to pay his own tribute to Leonard on Twitter. He encouraged fans to join him to remember the late actor on the social networking site.
“So maybe tomorrow we come together here and celebrate his life,” he continued.
“So let’s spend some time tomorrow celebrating Leonard’s life and remembering the man.”
As well as being co-stars, William and Leonard were close friends, with the late actor even serving as best man at William’s third wedding in 1997.
On Saturday, William was pictured in tears as he was seen for the first time since Leonard’s passing.
And he admitted on Twitter that he had been overwhelmed by the support he had received from fans on Twitter.
“I am so humbled by the worldwide outpouring of love that has been displayed; words cannot express my feelings.”
William previously released a statement about Leonard’s death, after the actor died following a lengthy battle with chronic obstructive pulmonary disease.
“I loved him like a brother. We will all miss his humour, his talent, and his capacity to love,” William told JustJared.com.