FRANKFURT (Reuters) - Drugs and chemicals maker Merck KGaA (>> Merck KGaA) expects its credit ratings to remain at investment grade levels following its planned acquisition of Sigma-Aldrich (>> Sigma-Aldrich Corporation), its finance chief said.

"We will face a downgrade by the credit rating agencies, however this downgrade will be moderate," Merck Chief Financial Officer Marcus Kuhnert said during a conference call on Monday.

"We will absolutely maintain our solid investment grade rating," he said.

Drugs and chemicals maker Merck earlier on Monday said it agreed to acquire Sigma-Aldrich for $17 billion (10.41 billion pounds) in cash to boost its Merck Millipore lab supplies business in the biggest takeover in the German group's history.

(Reporting by Ludwig Burger; Writing by Maria Sheahan; Editing by Arno Schuetze)

Stocks treated in this article : Sigma-Aldrich Corporation, Merck KGaA