ITV1 and BBC One fell to their worst audience shares ever last week, after the Channel 4 ratings success of Big Brother, Lost and cricket coverage.
Channel 4's Big Brother final drew a peak audience of 7.8m viewers
BBC One drew a 16.4% low of the total TV audience to Mind Your Own Business on 10 August while ITV1's share slumped to 11.2% during its daytime schedule.
"It was an incredibly competitive week for TV," a BBC spokesman said.
And ITV told Broadcast magazine: "ITV1 remains the most popular commercial broadcaster in the UK."
Last Friday the finale of Channel 4 reality series Big Brother drew a peak audience of 7.8 million viewers, representing 43.4% of the total audience at that time, according to unofficial overnight industry figures.
Two days earlier the channel drew an average of six million viewers to its debut episode of US drama Lost, while its daily coverage of the Ashes cricket drew regular audiences of 2.4 million.
Channel 4's success contrasted with that of ITV1, whose long-running series This Morning drew an audience of only 500,000 (10.7%) on 10 August.
Source: Broadcast magazine
However, ITV's spokeswoman said: "In terms of last week's performance, ITV1's viewing volume was up by 2% year on year."
Overall BBC One had an 21.4% share of the total viewing audience last week, while ITV1 had 18.5% and Channel 4 took 13.7%.
The BBC spokesman said the corporation's audience share had been hit by "so many major events taking place across all TV channels".
"There must have been fights up and down the country over the remote control," he said.
"At the end of the day, the viewers were the winners."