The singer has been absent from promotional performances and interviews over the past few weeks after being checked into a clinic for "unconfirmed reasons" back in October.
Despite joining the girls for tour rehearsals earlier this week, Sandford will not be travelling with them to promote their new album On Your Radar.
The group's representative told the Daily Mail: "Frankie is still ill and she's taking her time to recover. Under doctors' orders she is only permitted to attend tour rehearsals.
"She's easing herself back in to things with the group but will be appearing on the tour which starts at the end of this month."
Rochelle Wiseman previously told Digital Spy: "No, we're not splitting. She's not very well and you can't even have a flu or a runny nose before people start saying you've left [the group]."
The Saturdays will play their first headline UK arena date at Bournemouth's BIC on December 2.
The group's new single 'My Heart Takes Over' is available to download now, while their third LP On Your Radar will be released on November 21.
> The Saturdays release 'My Heart Takes Over' dance remix - listen
Watch the music video for The Saturdays' 'My Heart Takes Over' below: