The group were scheduled to appear at this Thursday's Scottish Royal Variety Performance in Glasgow.
It would have been their first live performance since Buchanan was replaced in the band by Jade Ewen, the UK entrant in the 2009 Eurovision Song Contest.
However, a statement on the band's official website reveals that the show has now been cancelled.
"Following the recent and well documented change of line-up to Sugababes, there are a number of legal issues to be finalised by ...
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