A 70 year old male who has been chewing tobacco for the past 50 years presents with a six months history of a large, fungating, soft papillary lesions in the oral cavity. The lesion has penetrated into the mandible. Lymph nodes are not palpable. Two biopsies taken from the lesion proper show benign appearing papillomatosis with hyperkeratosis and acanthosis infiltrating the subjacent tissues. The most likely diagnosis is -