Analysts say spending on online adverts is set to slow, casting doubt over Google's prospects as it gears up for its stock market launch.
The IPO had been expected later this month
Online ad sales will grow by just 11% in 2009, down from 65% in 2003, according to Jupiter Research.
Jupiter's report spells trouble for Google, which makes most of its money by selling targeted adverts linked to key words used in web searches.
It also looks set to add to uncertainty over the firm's plans to go public.
Google's stock market launch, expected later this month, is one of the most eagerly anticipated initial public offerings (IPO) in recent years.
Google hopes to raise up to $3.3bn (£1.8bn) in a listing that would value the company at more than $36bn.
But in the run-up to the IPO, some analysts have criticised the way the firm is handling its market debut.
They say the company's plans to sell its stock directly to the public in an online auction, intended to put small investors on an even footing with major financial institutions, may distort the price of the shares.
The Jupiter Research report will add to separate concerns that the firm's strong growth record is coming to an end.
Worries on that score were sparked last month when Google revealed that its sales grew by just 7% in the three months to June, falling well short of the double-digit increases it had reported in every previous quarter since 2002.
Some analysts say the firm's deteriorating sales performance does not justify its target price of between $108 and $135 per share.
The US stock market's recent weakness may also sap investors' appetite for newly issued shares.
On Friday, the Dow Jones index of leading US shares closed at its lowest level so far this year, weighed down by fears that the economy may be on the verge of a fresh slowdown.
According to a report in the UK's Observer newspaper, Google's advisers were meeting at the weekend to discuss postponing the market launch until investor sentiment improved.