William Shatner will wonder what his rift with Leonard Nimoy was about until the day he dies.

The 87-year-old actor is unsure why he and his 'Star Trek' co-star - who he once considered a best friend - fell out before his pal's passing in February 2015 aged 83, and he still questions why his late colleague couldn't have spoken to him about the feud.

According to the New York Daily News, William - who played Captain James T. Kirk in the sci-fi series - says in new book 'Live Long and... What I Learned Along the Way': ''Until the day I die, I will still wonder what caused that rift.

''After all the years we had spent together, after the bond we had forged, what could I have done that was so irreparable that he couldn't even talk to me about it?''

William admitted in 2016 that their fall out was ''sad'' and ''permanent'', and one guess about what the cause could've been was Leonard's refusal to participate in a movie he was making.

When asked why Leonard - who portrayed Spock in the 'Star Trek' film franchise - wouldn't appear in the film, William replied: ''I don't know. I thought he was joking at first and treated it as a joke because he sometimes would pretend and say, 'No, I'm not going to do that' and then say, 'Yes.'

''So that's what I thought he did.

''But that time he really meant, no. I just don't know, and it is sad and it is permanent. I don't know why he stopped talking to me.''

William didn't attend Leonard's funeral in 2015 due to charity commitments in Florida, which sparked an angry backlash from friends.

He tweeted on Saturday (28.02.15): ''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.

''I feel really awful. Here I am doing charity work and one of my dearest friends is being buried.

''I chose to honor a commitment I made months ago to appear at a charitable fundraiser. A lot of money was raised. So here I am; tell me off.(sic)''