'Morning Sun' singer Robbie sensationally rejoined Take That - whom he had originally left in 1994 - in July, but the reunion has not been planned as permanent.
Take That singer and songwriter Gary Barlow explained to Radio1 host Chris Moyles: "We agreed this when we started, it's 20 years since we begun this, for all of us, and to do a one-off album and a one-off tour would be a great thing.
"We said we'd only release it if it was a great thing, and it is a great record. I think once we've done that, we'll go back to being a four and Rob goes back to being Rob."
However, Robbie - who duets with Gary on his latest solo single 'Shame' - was more optimistic about a longer-term future with the group, which also includes vocalists Howard Donald, Mark Owen and Jason Orange.
He said: "The great thing is we've got options. The one underlying thing is music, and the music is great and we're not led by other stuff.
"It's a lot less work for me doing videos as well, I get to go to sleep while Mark or Howard do their things.
"The thing is, the door is open to anything. We've got an 18 month plan and whatever happens after that, happens after that. We're a bit undecided."
Although the pair couldn't give away many details about the album or forthcoming Take That tour, when asked if he will learn the songs from the two albums Take That have released without him, Robbie said: "not necessarily".
'Shame' will be included on Robbie's 'In And Out Of Consciousness: Greatest Hits 1990 - 2010', due to be released in October.