The The BBC's share of the national television audience slipped last year with teenagers switching off in "disproportionately large numbers".
The results were revealed by the corporation on Friday in its annual report for 2005/2006 .
The percentage of people watching at least 15 minutes of BBC television programming, the BBC's measure of audience "reach", fell last year to 85.3 per cent, down from 86.6 per cent.
Radio audiences remained stable, with weekly reach down just 0.1 per cent at 66.5 per cent.
But Mark Thompson, the BBC director general, said he was sure that the corporation's overall audience would prove to have grown if the numbers logging on to the corporation's website were included in its official reach data.
The number of users accessing bbc.co.uk surged to a weekly average reach of 12.3 million compared to 8.7 million the previous year.
The BBC received 3.1 billion pounds of income from 25 million licence payers last year and, as a consequence, audience share is a key measure for measuring its success.
Last month, Thompson appointed Andy Parfitt, Radio One Controller, to look at ways of boosting the BBC's offering for teenagers.
The decline in television audience coincided with above inflation rises for many of the corporation's senior executives as part of a two-year programme to bring their salaries up to market levels.
Thompson's salary jumped by more than 10 per cent to 619,000 pounds, angering staff unions who saw 1,100 jobs go last year in an efficiency drive with a further 2,000 due to be lost over the next 12 months.
Unions said they would be meeting on Monday to consider industrial action over proposed changes to the BBC pension scheme and a sub-inflation pay offer of 2.6 per cent.
Staff face further changes under a major revamp of the corporation under its "Creative Future" plan which Thompson said he hoped to unveil on July 19.
The reorganisation is designed to make it easier for the BBC to provide its programming and content on demand across television, radio, internet and other digital platforms.