The FTSE-250 company saw its shares hit a new low of 79p, down from 108p. The share price was the lowest the shares had been at since 1998.
In the first five months of 2001, Aegis Group's revenue grew by over 8% on a constant currency basis. This level of revenue growth is significantly less than that achieved last time.
In order to mitigate this, Aegis has implemented a programme of cost reduction initiatives across the group.
Yesterday was a bad for the sector as a whole, although rivals were not able to match Aegis' plunge. Cordiant Communications was down 7.1%; Tempus Group fell 0.9%; and WPP Group dropped 4.5%.
In a statement released to the stock exchange, the company said, "Aegis has seen reductions in marketing budgets from a number of clients. This is particularly marked in the US, where there has been a widely reported slowdown in the economy, most noticeably in the high-tech sector. European adspend, which saw some tightening in the spring of 2001, declined further in May."
Aegis, which owns the Carat media planning and buying network, said the start of 2001 had been good for Carat with $850m (£605.4m) of net new billings, a figure that reflected a particularly strong new business performance from its US operations.
The bulk of this comes from the appointment to handle Philips Electronics $600m (£428.1m) consolidated global media account following a three-way pitch in June
The London office of Carat, an incumbent on the business, triumphed in a pitch against Initiative Media and MediaVest New York, which was also an incumbent on the account.
Net new business for the first six months of 2001 is expected to be in excess of $1bn (£714m), in line with the corresponding period last year.
Carat's strong 2000 new-business performance is beginning to make a positive impact in 2001. However, it said the current market conditions are testing.
Additionally, Carat's operation in the troubled economy of Argentina, which since its acquisition in 1999 has been a strong contributor to profits, is expected to make a loss in 2001.
Aegis Research, which has rapidly expanded its global presence to 38 countries, has been less affected by reductions in marketers' budgets.