Former vice president Al Gore has been cleared of allegations that he groped and assaulted a masseuse in a Portland hotel room in 2006.

After a four-week investigation that included interviews with Gore, the masseuse, her acquaintances and hotel staff, authorities said there was no basis for prosecution.

Senior deputy district attorney Don Rees cited "contradictory evidence, conflicting witness statements, credibility issues, lack of forensic evidence and denials by Mr Gore".

Rees also said the masseuse and her attorneys were uncooperative, witnesses could not remember anything unusual, and that the masseuse failed a polygraph examination and would not say whether she was paid by a tabloid newspaper for her story.

"Mr Gore unequivocally and emphatically denied this accusation when he first learnt of its existence three years ago," spokeswoman Kalee Kreider said. "He respects and appreciates the thorough and professional work of the Portland authorities and is pleased that this matter has now been resolved."