More than £1.8bn was wiped from the value of the company as shares fell 127p, or 15%, to 726p.
The London Stock Exchange is said to be investigating whether the group contravened its rules by releasing price-sensitive information without making it public.
Reuters said it was mystified by the fall, and said it had given "no profits warning or a profits forecast and there is no price sensitive information which it should announce".
Nevertheless, after the meeting several analysts downgraded their profits forecasts for Reuters, which next reports its results on 21 October.
At the meeting, Reuters reportedly told analysts that "for the first time there are clouds on the horizon" of its Instinet off-market share trading system.
Doug Atkin, Instinet's chief executive, cited declining volumes on the Nasdaq market, competition from rival trading systems like Island, and Y2K worries as likely to depress earnings.
Instinet contributes 25% to Reuters profits.
Profit downgrade
John Holden of HSBC told his dealers that he was lowering the profits forecast for Reuters by £10-£20m and changing his recommendation from buy to hold on Instinet worries. Salomon Smith Barney also lowered its profit forecast by £20m to £615m.
The fall reflects the nervousness in the market about technology stocks, which were hit in New York over remarks by the president of Microsoft, Steve Balmer, that they were "wildly over-valued".
Meg Geldens of Merrill Lynch said that while the issues highlighted at the briefing were not new, they had drawn attention to the lack of positive news about the company's prospects.
Reuters' main business is supplying financial information to dealers and other traders. It also runs wire services and other news services for journalists.
Reuters news goes mobile
(19 Jul 99 | The Company File)
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