Renault and Nissan both announced leadership changes last week, seeking to reboot their alliance which was thrown into crisis last year by the arrest of Ghosn in Tokyo on financial misconduct charges, which he denies.
But the companies are struggling amid a global slowdown and a fall in emerging market demand, with pressures also coming from tougher emissions regulations in Europe and the need to invest in electric and self-driving technologies.
Rivals including Daimler and French Peugeot-maker PSA are set to add to the picture next week in sales updates, while Swedish truckmaker Volvo on Friday reported a sharp drop in orders.
Renault said late on Thursday that sales were likely to drop between 3% and 4% this year, compared with its previous forecast for a similar outcome to 2018. It blamed difficulties in Argentina and Turkey in particular.
The company also said its operating margin was set to come in at 5%, versus a previous 6% goal, as it struggles to keep a lid on research and development costs.
Analysts, who said they had expected sales targets to be revised, added that the margin guidance, equivalent to a steeper-than-expected 500 million to 600 million euro cut to Renault's operating profit, was the main shock.
"Renault guiding down for the full year should not be a major surprise, although the magnitude is," analysts at Jefferies said in a note.
Renault shares dropped to a six year low of 46.7 euros and at 0955 GMT were down 12.3% at 48.11 euros.
While Renault is not alone in suffering from an auto market downturn, its capacity to react was also thrown off kilter by the Ghosn turmoil, with industrial projects planned with Nissan placed on the backburner.
In its latest management revamp, sealed last week, Renault ousted Ghosn protegee Thierry Bollore as CEO and appointed financial chief Clotilde Delbos in the interim, billing the changes as a reset for the alliance.
Nissan also has new faces at the helm.
Delbos said on Thursday the two groups would now be able to focus on ways to cut costs after the shake-up, though the worsening backdrop for carmakers adds urgency for the alliance.
"Needless to say that this profit warning comes at a time of major instability at Renault and its partner Nissan," analysts at Evercore said in a note.
Carmakers also face billions of euros in fines if they fail to comply with EU emissions targets by 2021.
Many are behind the scenes thinking of further tie-ups with rivals to face these challenges, according to industry sources.
Renault has said in recent weeks that a failed deal with Fiat Chrysler, which withdrew a merger offer in June, was not at present on the agenda. Delbos said on Thursday that the logic of such a transaction was "as attractive" as before.
(Reporting by Sarah White and Sudip Kar-Gupta; Editing by Mark Potter and Alexander Smith)
By Sarah White