Some of the channel's top shows, including 'Big Brother' and 'Hollyoaks', are being simulcast on its website, along with the TV ads booked to run in the programmes.
The IPA asked the broadcaster to indemnify clients and agencies against potential legal action from third-party rights holders.
"If Channel 4 is going to broadcast TV ads online, it needs to ensure all rights are cleared or provide indemnity," Marina Palomba, the IPA's legal director, said.
A Channel 4 spokeswoman said: "It is our view that simulcast represents an extension of our TV service."
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