The Del Boy actor said that the death of creator John Sullivan convinced the cast that there would be no more episodes of the BBC sitcom.
A special Only Fools and Horses sketch will air on Sport Relief on Friday (March 21).
Speaking on The Michael McIntyre Chat Show, Jason said: "As you know, unfortunately, bless him, John Sullivan, the creator of it all, very unfortunately died.
"And as Nick Lyndhurst used to say, Sullivan made the ammunition and we just fired it. So he was the brains behind the whole thing. So when dear John died, we thought that was the end of it."
The 74-year-old also revealed that it was David Beckham who came up with the idea of making a cameo appearance in the Sport Relief sketch.
"He said he had always loved Only Fools And Horses and that it would be great to do something with me and Nick. So then the grapevine started to work as these things do," he explained.
Jason added that he found it easy to play Del Boy again, after he reunited with Lyndhurst for a run-through.
"Once I read the script that they intended to do, and I said yes I would do it, it was so much of Sullivan and so much of Only Fools and Horses the original, that I said, 'Yes, let's do it'.
"When I got together with Nick for the first run-through it came back like that [clicks fingers]. I didn't even have to think about it.
"Its like anything, its like falling off a log. As soon as I put on the gear and started to look at the script, back it came," he said.
Jeremy Clarkson and Abbey Clancy will also appear on the second episode of The Michael McIntyre Chat Show, which airs tonight (March 17) at 10.35pm on BBC One.