The , which acquired video-sharing site for $1.65bn (£884m) last month, will pull in around £900m of ad revenue in the UK by the end of the year according to Duncan, in comparison with Channel 4's estimated £800m.
He said that traditional media needed to wake up to growing threat of online ad revenue.
Google, which recently recorded a 70% rise in its third-quarter revenues to $2.69bn, received a visit from Duncan at its California headquarters last month. According to reports, discussions took place about making Channel 4's programme content available through the portal.
Duncan told the Financial Times: "People need to wake up and realise that this is not just a cyclical issue. There is a deep structural change taking place.
"If we want to protect the fantastic legacy of UK broadcasting, we need to wake up to this sooner rather than later."
Channel 4 is currently lobbying the UK government to allow it to have free access to digital spectrum, claiming it would be hampered in the public service broadcasting arena without the support.
Online ad growth has risen by more than 40% in the UK in the first half of the year to £917.2m, and is now just a percentage point behind national newspapers in terms of market share.
Google announced yesterday that it had acquired , the private company that develops collobrative wiki technology, and its sister sites, for an undisclosed sum.
The acquisition of JotSpot, which was founded by former Excite founder Joe Kraus, is another step in the broadening of Google's business.
Earlier this year, JotSpot released a new version of its wiki technology that makes shared pages similar to spreadsheets, photo albums and other software allowing people to better work collaboratively.
JotSpot currently has 30,000 paying users in more than 2,000 companies. However, it has many more people who use the its free service, which comes with sized-based restrictions.
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