The move is believed to be in recognition of the fact that Channel 5 has increased its market and audience share. The channel had an audience share of 5.8% last year, but managed to beat Channel 4 in parts of its daytime schedule.
The cash boost is also seen as a vote of confidence in the channel's management, led by chief executive Dawn Airey, who recently turned down a shot at the top Channel 4 job to remain at the smaller channel.
The channel will be the only commercial analogue terrestrial broadcaster to receive an increase in its programming budget. ITV and Channel 4 have cut their budgets in response to the advertising downturn.
Channel 5 currently operates on a budget of £150m, about one third that of Channel 4. Channel 5 launched in March 1997 with a budget of £110m, however its growth is believed to have been stunted somewhat by wrangling between United and RTL because the former has been reluctant to invest in the channel.
United is understood to be willing to sell its 35% stake in the broadcaster -- it has gradually been disposing of its TV interests since it exited ITV in 2000 -- but it is said to be holding out for good price.
In September, there were reports that Channel 5 had been in talks with News Corporation chairman Rupert Murdoch about the media behemoth, which controls BSkyB, taking a stake in the company. However, these talks appear to have come to nothing.
Another option would be for RTL to consolidate its ownership of the channel by buying United's share. However, this would scupper any plans Germany's Bertelsmann might have to take over Carlton or Granada, ITV's main partners, because of limits on UK media ownership rules.
Bertelsmann is currently in the process of buying out RTL's minority shareholders to take full control of the company.
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