An emotional Patrick Stewart said he was "deliriously happy" to be returning to Star Trek a generation later, telling San Diego Comic-Con that "you cannot have too many franchises in a career."

Stewart will reprise his fan-favorite role as Jean-Luc Picard in CBS's Star Trek: Picard after almost two decades, and presented unseen footage to a packed theater at the world's biggest pop culture fan convention. 

"We never know, do we, when our best moment will be - and that is now," he said.

His voice choking, Stewart described battling overwhelming emotions when shooting the final scenes of Star Trek: The Next Generation years ago, and how he had felt reluctant to return to the sci-fi franchise until meeting a writing team including Pulitzer-winning author - and now showrunner - Michael Chabon.

"As soon as we began talking in the writers' room, that powerful emotion came surging back. And it's still right here on this platform and it's not going to go away," he said.

The trailer revealed the surprise return of other popular characters including Data (Brent Spiner) and Seven of Nine (Jeri Ryan) who then joined the cast and producers on stage.

It showed an age-worn, soul-searching Picard living back on Earth on his vineyard, and struggling to reassimilate. 

A mysterious young woman comes to him for help, before a few brief clips showing him seemingly back aboard a spaceship. 

Jonathan del Arco and Jonathan Frakes will also return, it was announced.

Star Trek, a multi-billion dollar cultural phenomenon beloved by fans worldwide, turned 50 in 2016.

The Next Generation ran from 1987 to 1994 with 178 episodes, and spawned several films, the last in 2002.

The new show is not a direct sequel, and will be more character-driven, said executive producer Akiva Goldsman.

Stewart recalled how his children explained to him what Star Trek was when he was first approached as a stage actor in the 1980s.

"I'm deliriously happy that now I have four grandchildren who have the same attitude to what we're doing," he said, describing how his grandson Mason had begged to be allowed to see the trailer first. "That gave me great satisfaction."

Addressing his decision to return after years of dismissing the possibility out of hand, the X-Men star said: "I knew that something very unusual was going to happen. I wanted to be a part of it."

"To imagine that something like this might go ahead and I wouldn't be there, would have been just too defeatist."

Picard will be released in 2020.