Express Scripts, one of the largest US pharmacy benefits managers, has launched a hostile bid for rival Caremark Rx as it refuses to abandon its battle to wrestle the company away from drug store chain CVS.

Express Scripts, which is offering $25bn (€19.4bn) in cash and stock for Caremark, said it would start an exchange offer for all outstanding shares of Caremark Rx. Later today the company will file a full plan about its intentions with the Securities and Exchange Commission.