Provisional data for the first three weeks of the new year suggests that viewing figures have fallen by 30% among its target audience of 16- to 34-year-olds.
The figures for the full month are due to be released by audience research bureau Barb on Tuesday this week.
The new figures are the first to be based on results using a new selection panel of viewers, who are meant to reflect more closely the age and mix of the population.
The figures will put into question Channel 4's spending on a number of American shows, including Friends, Sex and the City and ER, which are designed to attract 16- to 34-year-olds.
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