UBC will hold on to the remaining 49% stake and work together with Channel 4 to develop Oneword. Board positions will be split equally between the partners. The companies say that initially there are no plans to significantly increase investment in the station.
Oneword broadcasts on the Digital One national digital multiplex. It holds the only national commercial speech licence and was the Sony Radio digital station of the year for 2003.
Over the past year, the station has almost doubled its listeners to reach more than 112,000 people.
Michael Hodgson, head of corporate development at Channel 4, said: "As we move towards a fully digital and converged world, we are actively exploring opportunities that exist across all digital platforms and UBC Media will be a valuable partner to help us further explore the radio sector."
UBC is a radio and TV content producer and also owns the Classic Gold Digital station and provides digital radio data services. It took full control of Oneword in July last year by buying the 50% stake belonging to its then partner USI Holdings.
UBC has a 10% stake in Celador Radio Broadcasting, which is bidding for FM licences in Manchester and the Solent.
Simon Cole, chief executive of UBC, said: "Channel 4's investment is an important vote of confidence in the prospects for digital radio in the UK and in UBC's position in that marketplace."
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