BBC1 had a particularly rough day on Wednesday 25 July, when the National Lottery show Jet Set 2012at 8pm attracted just 1.8 million/9.2% share and Vet Safarihalf an hour later got 1.7 million/8%.
Also failing to make much of an impression was Would I Lie to You?at 10pm on Saturday 28 July with just 2.4 million viewers (13.4%). The penultimate episode of drama True Dare Kisson 26 July averaged 2.7 million/12.6% share in a 9pm slot.
The channel's best performers that week were old favourites. EastEndersbroke the 9 million mark on 23 July and the 10 O'ClockNewson the same day gained an impressive 6.2 million/31.1%.
BBC1 has dropped below the 21% mark on two other occasions this year. In the week beginning 9 July it managed just 20.5% and at the beginning of April it had a share of 20.6%.
A BBC spokeswoman said: 'All channels suffer with changing viewing habits over the summer and we're confident in the performance of the channel in the year to date.'
Over the year so far BBC1 is still the most popular channel across all hours with an average share of 22%. ITV1 is the second most popular with a share of 19.1%.