The BBC's share of total TV viewing fell to around 35 per cent this year, latest figures have revealed.
BBC1 and BBC2 recorded the sharpest ratings declines this year of the five main public service networks, of 6.1 per cent and 6.3 per cent respectively between January 1 and November 14, compared with the same period last year.
But ratings for most of the BBC's digital channels continued to grow.
BBC3 and BBC4 both had their best months yet in October, in terms of average weekly reach.
The corporation's digital TV services - BBC3, BBC4, CBeebies, CBBC, BBC News 24 and BBC Parliament - has just under three per cent of total viewing, compared to 2.3 per cent in October last year. The audience share for all the BBC channels for the year to November 14 stands at 35.2 per cent - down from 36.8 per cent in the same period of 2004, a year-on-year fall of around four per cent.
The average weekly reach for all BBC channels for the year to date is 87.9 per cent - down from 89.9 per cent in 2004.
ITV1 has seen its audience share fall by 5.7 per cent.
ITV's ratings decline have for several years been steeper than BBC1's.
Channel 4 is the only one of the five main networks not to see its audience share decline in 2005, but instead retain its 2004 audience share of 9.9 per cent in the year to date.
Last year its share grew from 9.6 per cent in 2003 to 9.8 per cent.
Channel Five saw a slight decline in audience share from 6.58 per cent to 6.5 per cent for the year to date.
A BBC spokesman said BBC1 and BBC2 had performed well in the multichannel climate.
He said: "BBC1 and BBC2 have held up incredibly well in such a competitive environment. As audiences fragment and consumption changes, reach and appreciation are absolutely critical.
"What is most important is that the BBC's portfolio of services and channels work as a whole to reach everybody - through radio, TV, online and interactive.
"It's not just about the performance of individual channels, and nor is it just about share; to take these in isolation tells you only part of the story."
ITV1 said its performance would improve by the end of the year thanks to I'm a Celebrity . . . . Get Me Out of Here! and a raft of new dramas.
So far this year, ITV1 has averaged a 29 per cent peak time share in all homes, which it says is four points higher than BBC1.
Viewing of multichannel services such as ITV2, Sky One, BBC3 and E4 continues to grow, hitting 29.5 per cent, compared with 26 per cent in the same period last year.