Swiss drugmaker Roche says it will end its offer to acquire U.S. diagnostics firm Illumina Inc. for about $6.5 billion after forecasting that Illumina shareholders will reject its dissident slate of directors.

Roche started an offer to acquire the San Diego company's shares for $44.50 each in January. It then raised that to $51 last month. But Roche says it decided not to extend the latest offer past an April 20 expiration.

Two shareholder advisory firms have recommended that Illumina shareholders reject Roche's offer.

Roche says it does not believe a higher bid is in the best interest of its shareholders without getting access to Illumina's books.

Roche had proposed a slate of directors, but it said preliminary estimates indicate that Illumina's incumbent directors will be re-elected.