The true cost of BBC One is £1.4 billion, the corporation will reveal next week, nearly £600 million more than it was prepared to admit last year.

The increase will be shown in the public broadcaster’s latest set of figures, which demonstrate that, despite rising spending, it is winning over fewer viewers and so becoming poorer value for money.

After the departure of Michael Grade as chairman of the corporation, the BBC Trust has decided to include the costs of news gathering, marketing and other overheads for the first time in the BBC One budget. A year ago the BBC said that its spending on the channel was £840 million. On the revised basis the figure would have been £1.295 billion.

The figure shows how the BBC can dwarf its principal commercial rival and helps to explain why the corporation can afford to pay Jonathan Ross £18 million over three years. His salary and that of other top stars, ranging from Sir Terry Wogan to Gary Lineker, will be the subject of an inquiry to be announced by the trust when this year’s annual report is published on July 3.

A precise comparable figure for ITV1 is not available, but it spends about £1.1 billion, about £300 million less than its rival; its expenses include tax payments and the cost of advertising sales staff, neither of which are paid by the BBC. Despite the additional cash BBC One’s share of viewing fell from 22.9 per cent in 2005-06 to 22.3 per cent in the year to March 2007, and it has fallen further since, averaging 22 per cent this year.

All leading broadcasters in Britain have been suffering from falling audiences as viewers switch over to digital and benefit from a greater choice. While ITV and Channel 4 have had to contend with falling revenues amid weak advertising, the licence fee income of the BBC increased last year by 4.6 per cent to £3.24 billion, reflecting the former agreement of 1999. Between now and 2012 the household bill will go up by below the rate of inflation.

The BBC Two budget will be declared at £563 million – nearly £200 million more than disclosed last year – an increase of £11 million on the previous year on a comparable basis. Channel 4 spent £643 million last year on its core channel.

The figures will show that BBC Three spent £117 million, generating only 0.7 per cent of the audience, and the cost of BBC Four was £61 million for 0.2 per cent of the audience.

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