The late star's fears are revealed in a previously unseen letter he wrote to his wife in March 1964, three weeks after their whirlwind wedding.
The note, written while Ekland was abroad filming a movie, is part of a private memorabilia collection which is set to go to auction in Wiltshire, England next month (15Aug09).
Sellers confesses his worries that Ekland would have an affair while she was away, admitting he couldn't understand why such a beautiful woman would want to be with an ordinary man like him.
He writes, "I have a dreadful fear that you might leave me. I love you so desperately, that I find it very difficult to understand why you married me. You who are the most lovely thing in the whole world. What do you see in me? I'm not handsome. I'm not tall. I'm not special in any way. Without a doubt I am a raving idiot and I ought to have my head examined.
Ekland was filming Guns at Batasi with John Leyton and the pair had kissed at a previous screen test - and Sellers was terrified his wife might leave him for the handsome actor.
He adds, "I thought of something an actor once said to me, that he always had to become involved with the woman he worked with otherwise it didn't look real. The thought of this made me break out in a cold sweat and want to be sick.
The couple went on to have a daughter, Victoria, but divorced in 1968 after four years of marriage.
Sellers died in 1980 aged 54 after suffering a heart attack.