Mr Anwar has already seen a sodomy conviction against him overturned
Malaysian opposition leader Anwar Ibrahim claims he has medical proof that allegations he sodomised a male aide are false.
A doctor's report apparently ruling out sodomy has surfaced on Malaysian news websites.
Mr Anwar told reporters the report proved "that the allegations... are baseless and politically motivated".
He has accused the government of trumping up the claims to try to check the rise of his opposition movement.
Mr Anwar faced a similar accusation a decade ago when he was deputy prime minister.
Sodomy is an offence punishable with a jail term in mainly Muslim Malaysia, and Mr Anwar served six years of a prison term before the conviction was overturned by Malaysia's Supreme Court.
The report - in scrawled handwriting on a hospital form - was purportedly written by the doctor who examined Mr Anwar's accuser, now 23, at Pusrawi Hospital on 28 June.
In it the doctor appears to conclude that there is no physical evidence of sodomy.
Speaking to reporters, Mr Anwar demanded that police stop their investigation immediately, and accused them of involvement in a political conspiracy against him.
"I condemn in the strongest terms their negligence, dishonesty and recklessness in humiliating the nation by dragging us all through this vile and filthy charade," he said.
Hospital officials have neither confirmed nor denied the authenticity of the report. The doctor who authored the document has gone on leave, Mr Anwar is said to have told reporters, citing hospital authorities.
Mr Anwar was barred from standing for public office until April.
His People's Justice Party has suggested he could run for parliament in a number of possible vacant constituencies.
Should he win, it would pile further pressure on Prime Minister Abdullah Badawi, who is already feeling the heat from poor election results and high fuel prices.
The opposition alliance now needs just 30 more seats to win a simple majority and form a government.