Internet firms such as Yahoo! and Excite face a black hole in advertising revenue as companies make swinging cuts to their promotional activity in the wake of the World Trade Centre disaster.

Figures published yesterday indicate that the number of new advertisements bought on the web fell by a quarter in the week of the attacks and were down 16% last week.

The sudden collapse is likely to put fresh pressure on the finances of loss-making dot.coms. It will raise new questions about the business model of firms such as Yahoo!, which gets 90% of its revenue from website advertisers.

Charles Buchwalter of market research firm Jupiter Media Metrix, which carried out the research, said: "Advertisers clearly weren't in the right frame of mind to initiate new campaigns. They're trying to figure out what to do now."

On September 12, the day after the terrorists struck, Jupiter Media Metrix said 753 new online advertisments were purchased, compared with 1,565 the previous week. Yahoo! is the biggest online host of advertisers, followed by MSN, iWon, Netscape and AOL.

Yahoo! declined to discuss its advertising revenue, except to say it did not expect a material impact from the disaster in its third-quarter results, due next month.

Reports yesterday suggested that some websites are cutting their prices for ad space by as much as 80%.

Dot.com shares fared better than most in last week's stock market rout, but Yahoo!'s share price dropped by 13% on Friday, as analysts began highlighting their concerns about online advertising.