Merriam-Webster Collegiate Dictionary - due to

 preposition  Date: 1897 as a result of ; because of ~ the complaints of uptight parents…he lost his job — Herbert Gold Usage:  The objection to ~ as a preposition is only a continuation of disagreements that began in the 18th century over the proper uses of owing and due. Due to is as grammatically sound as owing to, which is frequently recommended in its place. It has been and is used by reputable writers and has been recognized as standard for decades. There is no solid reason to avoid ~.