Zurich Financial Services, the Anglo-Swiss insurance group, said earnings in the second half of the year were "not likely to reach the level achieved during the first six months".
The statement from Zurich, formed last week from the merger of Zurich Group with British American Financial Services, was echoed by Irish Life, the largest insurance group in Ireland.
Irish Life warned that market weakness could wipe out gains made on soaring investment markets in the first half of the year, which helped to boost operating profits by 40 per cent.
David Went, the managing director of Irish Life, said sales looked set to continue their upward trend.
"Sales performance in the second half looks set to continue the trends in the first half. However, our profitability is geared to the equity markets and the operating profit will be impacted by the course of investment markets since June.
"Based on current conditions, this is likely to result in a reversal of much of the investment gains."
Like other investment firms, Irish Life charges for managing money by levying a fee as a percentage of the investment.
When markets slump, fees will go down while overheads stay the same, creating the geared effect on profits.
The warnings may signal an end to the boom in financial services after a strong six months between January and June when surging markets created healthy conditions for the sale of financial products.
Zurich said its member companies - including Allied Dunbar, Eagle Star, Threadneedle Asset Management, Farmers and the existing Zurich - generated earnings of $1.42bn (pounds 850m) in the first half. That compared with $2.1bn for the whole of 1997, a record unlikely to be sustained.
Most of the growth came from Ireland. Operations in the US were hit by a fall-off in fixed annuities. US pensioners have been shunning the annuities because the income they get from them depends on long-term interest rates, now at rock bottom.