The group — which features the vocals of Londonderry song-stress Nadine Coyle — put a countdown clock on their website, marking nine days until Friday October 19.
Shortly after it appeared, Nadine told her Twitter followers: “Two hours in. How are you all feeling now? I have serious excitement that has taken over since last night! Xx”.
Girls Aloud — who celebrate their 10th anniversary next month — are expected to perform on Strictly Come Dancing, BBC1's rival show to Simon Cowell's X Factor, with band member Kimberley Walsh currently competing on the ballroom dancing show.
But the band, who formed on Popstars: The Rivals in 2002 and went on hiatus in 2009 to focus on individual projects, have not yet released details of how they will mark their return.
The countdown, with the word Ten above the clock, comes after the five-piece are said to have teamed up to film a video for their comeback single after recording the lyrics for the track separately.
Kimberley, Nadine and Nicola Roberts recently watched Cheryl Cole perform a London date on her solo tour, A Million Lights, while fellow bandmate Sarah Harding was said to be in a studio recording solo material.
Cole has previously hinted that the Sound Of The Underground singers will mark their 10th anniversary in November on “our birthday”.
The former X Factor judge tweeted: “Today is a very good day to be a Girls Aloud Fan! #GirlsAloudAreBack.”
The group are expected to release a greatest hits album in time for Christmas, as well as new material.
They are believed to have filmed a new video for a comeback single last week.
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