It may not be one of the great remaining mysteries, on a par with the nature of dark matter or the origins of the universe, but the question of how many women Warren Beatty, 72, has slept with certainly seems to have got New York's media-land in a froth.

Peter Biskind, Beatty's new biographer, estimates that the famously seductive star of Bonnie and Clyde and Reds has notched up 12,775 sexual conquests, including Isabelle Adjani, Diane Keaton and Madonna. If true, that is impressive. Don Giovanni could only claim a lacklustre 2,065, according to Mozart's librettist, Lorenzo Da Ponte.

Biskind writes in his book, Star: How Warren Beatty Seduced America, that he arrived at the figure by "simple arithmetic". He appears to have worked out the number of days between Beatty losing his virginity at 19 and the date in 1991 when he met Annette Bening on the set of Bugsy and fell into monogamy, and applied the questionable logic that during that entire period Beatty slept with an average of one woman a day. Biskind, an accomplished writer on Hollywood and author of Easy Riders, Raging Bulls, adds that for these purposes he ruled out "daytime quickies, drive-bys, casual gropings, stolen kisses and so on".

The exercise may not gain Biskind honorary membership of the American Academy of Arts & Sciences, but it has generated considerable publicity.

An account in the New York Post, headlined "Sexy tell-all jumps into Beatty's bed", bills the book as an authorised biography. However, even its publisher, Simon & Schuster, makes no reference to Beatty having co-operated with the author.

Beatty's lawyer, Bertram Fields, told the Huffington Post that Biskind's "tedious book was not authorised by Mr Beatty". He accused Biskind of writing "many false assertions" and of quoting Beatty "saying things he never said".

According to the Post's account, such quotations include Beatty telling Biskind of Jane Fonda – who apparently initially thought he was gay: "Oh, my God. We kissed until we had practically eaten each other's heads off."

Joan Collins allegedly stumbled exhausted out of bed after a session with Beatty exclaiming: "I don't think I can last much longer. He never stops – it must be all those vitamins he takes."