The 'Bodies' singer was spotted at New York's Electric Lady Studio on Saturday (26.09.09) with his former bandmates Gary Barlow, Mark Owen, Howard Donald and Jason Orange.
The group is reportedly working on new songs ahead of a reunion tour.
A source said: "He is back with Take That in the studio and they're all writing together again. Robbie wouldn't be happy performing old Take That songs that he has had no part in.
"But if they can pen new tracks that they're all happy with, they'll record them and he'll be back with them on stage."
It is the first time Robbie, 35, has worked with the 'Greatest Day' hitmakers since he quit the band in 1995.
The singer flew from Los Angeles to New York last Wednesday (23.09.09) to celebrate Mark Owen's stag party with the rest of the band at private members' club Soho House.
Three days later, the five men spent seven hours at the recording studio in Manhattan's West Village, leaving around 9.20pm.
Robbie is said to have watched Gary, Mark, Jason and Howard put the final touches on their upcoming live album, before sitting down to write new material.
The source revealed to Britain's Daily Mirror newspaper: "The lads really did want to celebrate Mark's stag do. But they thought it was the best place to hook up and start writing again without attracting too much attention.
"They are nowhere near recording anything. This was a writing session to see how they go."