The 40-year-old singer - who reunited with his former band in 2010 for their sixth album 'Progress' - is said to be worried about upsetting Gary, who he's had differences with in the past, and isn't sure he's ready to focus completely on Take That rather than his solo material.
A source revealed: ''Robbie isn't against the idea, although he doesn't really have time this year to commit to Take That, although they have collaborated on compositions.
''He also doesn't want to upset Gary, given that they patched up their differences and are as close as ever.''
The 'Let Me Entertain You' hitmaker has reportedly suggested to Gary, 43, that they take things slow by working on material together - along with fellow bandmates Mark Owen, Jason Orange and Howard Donald - for now, and then discussing a world tour at a later date.
The source told The Sun newspaper: ''He's suggested recording together, seeing how that goes then embarking on a world tour in 2015.''
Gary himself is preparing to kick-off his own solo tour with dates in cities including Glasgow, London, Cardiff and Manchester starting later this month, and Take That are already back in the studio working on a new album which is expected to be released later this year.