Underscoring the momentum of internet advertising, Mr Duncan said: "It looks like Google will extract £900 million in advertising revenue in the UK, overtaking Channel 4, which is by some distance the second largest ad funded TV company in the UK."

Channel 4’s annual advertising income is expected to come in at around £800 million.

Mr Duncan added: "People need to wake up and realise that this is not just a cyclical issue – there is deep structural change, rather like global warming. If we want to protect the fantastic legacy of UK broadcasting, we need to wake up to this sooner rather than later."

His comments come as Ofcom, the media watchdog, reviews Channel 4’s long term financial future.

A Google spokesman said that the company did not comment on future financial earnings. However, according to company filings, 15 per cent of the company’s first-half revenues come from the UK, accounting for some $700 million (£370m).

The explosive rates of growth seen this year by Google suggest that the figures cited by Mr Duncan are within its grasp. Earlier this month, the company announced that third-quarter profit nearly doubled to $733 million compared with the same period a year ago. Revenue for the period totalled $2.69 billion, a 70 per cent increase.

Google recently confirmed its intention to take on traditional broadcasters when it agreed to buy YouTube, the online video sharing site, for $1.65 billion in an all-share deal.

According to figures released by the Internet Advertising Bureau earlier this month, online now accounts for more than 10 per cent of overall advertising spending, up from 7.2 per cent a year ago, and is expected to match spending on television campaigns by the end of the decade.

According to the IAB, internet marketing grew by more than 40 per cent in the first half of 2006 compared with the same period a year ago, to £917 million.