Williams quit the British band in 1995 but reunited with them in 2010 after working with fellow singer Gary Barlow on the album Progress. He even joined them on tour between 2010 and 2011.
However, the Angels hitmaker has since gone on to work on more solo material, as well as welcoming his first daughter in 2012, and it is unclear how he will feature in the group's future plans.
Barlow has now given Williams until May (14) to decide if he will officially rejoin the group, telling U.K. chat show host Alan Carr, "We'd need to know what we're doing by the end of May... The thing with Robbie is he's family with us now. If he's not in the band he'll write on the record and you know we'll work with him on his next record. We do this now, we work together a lot. Whether he'll be in the final line-up I'm not sure."