Little Britain has been among the hits of BBC digital television

Recent press reports have said 55 days' worth of BBC digital programmes scored zero in the official ratings in 2003.

But, writing in The Guardian, BBC director of television Jana Bennett said digital was "coming of age" with shows like Little Britain and Balamory.

"BBC digital channels have grown remarkably quickly," she wrote.

The argument about zero ratings was "a red herring", Ms Bennett said, because most digital channels got them at times.

BBC One and Two are still the beating heart of BBC television Jana Bennett BBC director of television

"Anyone sad enough to study digital ratings knows occasional zeros are occupational hazards, especially in the small hours," she wrote.

ITV2, one of the most popular digital channels, registered 100 hours of zero-rated shows in December, she pointed out.

"No shame in that. The point is simply that digital is a different universe."

But, now that more than 50% of households in the UK have digital TV, ratings for channels like BBC Three and BBC Four are going up, she wrote.

Ms Bennett also said a recent raft of digital shows had enjoyed "critical acclaim and anticipatory excitement".

These included Little Britain and Nighty Night on BBC Three, The Alan Clark Diaries and National Trust on BBC Four and Balamory on children's channel CBeebies.

The Alan Clark Diaries starts on BBC Four on Thursday

"What we're doing here is something that's always been central to the BBC's purpose - boosting the take-up of new kinds of broadcasting with high-quality programmes that people really want to watch and listen to," she wrote.

But BBC One and BBC Two were still the "beating heart" of BBC television, she added.

Her comments come after Culture Secretary Tessa Jowell said the success of the BBC's digital strategy was central to its charter review - which will set out the corporation's objectives and obligations.

And it will be crucial to the government's plans to switch off the analogue signals which carry the five main channels in 2010.

"All of the public service broadcasters have a crucial role to play in bringing switch-over about," Ms Jowell said in a speech on Tuesday.