Police in the US state of Oregon have said former vice president Al Gore will not face charges over an alleged 2006 groping of a masseuse "due to a lack of credible evidence."

"After evaluating the materials submitted by PPB (Portland Police Bureau) I have concluded that I agree with the assessment that a sustainable criminal case does not exist," said deputy attorney Don Rees for Multnomah County in the northwest state, in a memo obtained by AFP.

"Upon completion of this investigation PPB is not recommending prosecution due to a lack of credible evidence," Rees said.

Oregon said earlier this month it had re-opened the four-year-old probe into the allegations, after an earlier inquiry stalled when the Nobel laureate's accuser, masseuse Molly Hagerty, declined to be interviewed by police.

Hagerty accused Gore, 62, of trying to have sex with her while he was in a hotel in Portland in October 2006, while on a visit to the state to promote his anti-global warming activities.

Rees detailed a number of instances when Hagerty's accusation failed to live up to the story, saying she had "not provided as repeatedly requested medical records" that she "failed a polygraph examination" and that forensic testing failed to show up any seminal fluid.

The deputy attorney also noted that it appeared Hagerty had been paid for her story by the National Enquirer tabloid, where the explosive allegations were first made.

Gore, winner of the Nobel Peace Prize in 2007 for his work on raising awareness of climate change, last month confirmed he was separating from wife Tipper after four decades of marriage.