French investment consultants have warned that Ixis Asset Management and Natexis Asset Management are under scrutiny from their clients and risk losing billions of assets as a result of staff departures and worries about their forthcoming merger.

The fund managers will combine in late November to create Natixis Asset Management – France's largest such group with €534bn ($676bn) under management. A consultant close to both said Ixis feared it could lose tens of billions of euros in assets as a result of the tie-up. The consultant said a substantial number of institutional clients had invested money with Ixis because it is AAA rated. Ixis is likely to be downgraded when it becomes Natixis, prompting clients to review the relationship. Ixis also manages billions of euros for France's state-owned treasury fund, the Caisse des Dépôts et Consignations, and its subsidiary CNP, France's largest insurance group. However, the Caisse des Dépôts et Consignations sold its 35% stake in Ixis' parent, La Caisse d'Epargne, for €7bn this year after a public row over the creation of Natixis. Consultants say it is only a matter of time before the assets come up for review – a tempting prospect for French and foreign fund managers. The Natixis merger is prompting senior staff to resign. Daniel Roy, chief executive of Natexis, left last week. He is understood to have gone after being sidelined for the top job at Natixis. A spokeswoman declined to comment on his departure. Dominique Piermay, leading partner at Fixage, the Paris-based investment adviser, said Natexis was "under scrutiny".