The company, home to popular serial drama Coronation Street and Simon Cowell's Britain's Got Talent, said its channels had not attracted the audiences it wanted in the first four months of the year, with its share of viewers falling to 21.6 percent from 23.4 percent a year ago.
Its share of viewing has been lower than expected so far this year, Chief Executive Adam Crozier said after Wednesday's trading update, adding that the company is confident in a "strong schedule to come", including the World Cup in June.
First-quarter advertising revenue rose 2 percent, but the company said that it achieved a 19 percent rise in April advertising spend thanks to a late Easter and would lift advertising revenue by 7 percent in May, with the World Cup expected to help it to a 12-15 percent rise in June.
ITV shares broadcasting rights for the World Cup tournament in Brazil with the publicly funded BBC.
Shares in ITV, which have risen 5 percent in past two weeks, fell 5 percent 181.6 pence by 0826 GMT, the worst-performing stock in the blue-chip FTSE 100 index.
Analysts at Citi said the second-quarter guidance for net advertising revenue was just short of its 14 percent forecast and that ITV had "some work to do for the remainder of the year to get back on the front foot" on audience numbers.
ITV said that total external revenue for the three months to March 31 rose 2 percent to 585 million pounds.
Revenue at its production unit fell 4 percent in the quarter, which it attributed to the phasing of programme delivery.
ITV has been building up its production capabilities to reduce its reliance on advertising revenue and last week bought U.S. reality TV producer Leftfield Entertainment for up to $800 million.
(Editing by David Goodman)
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