The new Sugababes line-up was announced last month
Sugababes member Amelle Berrabah has been admitted to a private health clinic suffering "from a severe bout of nervous exhaustion".
A message on the group's website said the 25-year-old had been advised to take a "short break" following medical advice earlier this week.
The singer's health problems have been attributed to the pressures surrounding the group's recent line-up change.
Original member Keisha Buchanan left last month in a flurry of controversy.
Members Heidi Range and Jade Ewen said in their joint message: "It seems that with all the recent events in the band right up to Jade joining us a few weeks ago, the stress over a period of months has unfortunately taken its toll on Amelle and she is now suffering from a severe bout of nervous exhaustion.
"For the next three weeks, Amelle will be staying at a private health clinic in Europe where she will be able to rest and recuperate under expert supervision."
Range said: "Both Amelle and I have been through a very traumatic time together in recent months and particularly over the past few weeks." Ewen added: "We're glad that Amelle is getting the right support now to help her return to work."
Berrabah is expected to return to the group "in full health" by the beginning of November.
The singer's absence means the group will have to cancel their first live performance together since the line-up change.
They had been due to perform in Leeds and appear on a German TV show this weekend.
The group also had to pull out of headlining the Scottish Royal Variety Performance last week due to "a number of legal issues to be finalised".
"Although we know this news all comes as a huge disappointment to Sugababes fans, we really do have to treat Amelle's welfare as our priority at times like this," Range and Ewen said.
After original band member Buchanan left the group last month, Range and Berrabah denied the 24-year-old's claims she was pushed out.
The group was formed in 1998 by Buchanan, Mutya Buena and Siobhan Donaghy.
Range replaced Donaghy in 2001 and in 2005 Buena was replaced by Berrabah.
Ewen is still re-recording vocals for the group's next album, which had featured Buchanan's voice.
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