Vanity Fair, which started on Sunday evening at a reported cost of around £1m per episode, was watched by only 6.9 million viewers, compared to the 9.9 million who watched ITV's long-running police drama, Taggart.

This compares to the BBC's adaptation of Jane Austen's Pride And Prejudice, which regularly attracted around nine million viewers with a repeat of the series drawing around six million.

Pride and Prejudice: Attracted audiences of around 14 million

But BBC bosses are reported to be playing down suggestions that, given the expense and fanfare surrounding the new series, the ratings are disappointing.

However, despite their support, the third and subsequent episodes of Vanity Fair are being moved to a later slot on Sunday evening so that they do not clash with a re-scheduling of ITV's police series Heartbeat.

There are fears that the mediocre audience figures for the adaptation of William Thackeray's novel could reflect a general malaise for costume drama.

Earlier this year ITV's Hornblower also failed to pull in as large an audience as expected, with just under nine million tuning in to its weekday evening slot.

Hornblower stars Robert Lindsay and Ioan Grufford

Other TV adaptations of classic novels, like Moll Flanders and Emma, managed to edge above the 10 million mark.

But a BBC spokeswoman defended the figures for Vanity Fair, saying the corporation was very pleased with the overnight figures.

She said the smaller audience in comparison with that watching Pride and Prejudice could be due to the fact that: "Pride and Prejudice is a much better known story. Vanity Fair is a difficult piece and not everbody knows it."

And she said the overnight figures did not take into account the number of people who had video recorded the programme and watched it later.

There have been some high profile complaints about the number of costume dramas on British screens.

Earlier this year, Culture Secretary Chris Smith told BBC governors he wanted to see the corporation produce more gritty, contemporary drama like Our Friends In The North.

"I would be worried if they ended up just doing Pride And Prejudices. Formulaic historical drama without any real bite isn't going to work with the viewers," he said.

But a BBC spokeswoman said period pieces were just part of what the corporation offered in the way of drama.

"We are not making them at the expense of contemporary dramas like Casualty, City Central or Cops," she said.

"They are at the more expensive end of drama but they can be well-made and well-produced and there will always be room for them in the schedules."