Capital bought its existing stake in October 2001. At the time, it agreed a date of September 2004 for an option that would allow it to acquire the rest of the station.
The decision to bring the date forward is being seen by industry experts as a reaction to Capital 95.8 FM losing the top slot to rival Heart in the latest set of Rajars for the third quarter.
Choice will be seen as a valuable addition to the Capital Radio portfolio in London, which already includes Capital 95.8 FM and specialist new-music station Xfm.
Capital Radio Group's total London reach will now become 31%, a share of 12.8% of all London listening.
Linda Grant, commercial managing director, said: "Choice delivers one of the purest youth audiences of any London station, and as such adds a key piece to our London brand portfolio jigsaw."
Choice FM was launched in South London on 96.9FM in March 1990. It extended its reach to North London with its launch on 107.1 FM in May 2000. It had 415,000 listeners, according to the Rajars for the third quarter of 2003.
Choice FM is the capital's only dedicated legal urban station and Capital said in a statement it fully intends to retain this position and its specialist music shows that cater for the different music genres, including hip hop, Rn'B and UK garage, that comprise urban music.
Graham Bryce, managing director of Xfm, will extend his responsibilities to include running Choice.
He said: "Our vision is to build Choice into London's leading urban music station, becoming the number one choice for young urban Londoners. Longer term, we intend to fully exploit the use of digital technology to build Choice nationally into the UK's leading urban music station and the number one urban music brand."
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