Ryan O'Neal and Bianca Jagger were caught having sex ''like a pair of maddened wart-hogs'' in the marital home she shared with Sir Mick Jagger.

A previously-unpublished letter from screenwriter Terry Southern to legendary filmmaker Stanley Kubrick detailed how the 'Dr. Strangelove' scribe accidentally walked in on the pair in the throes of passion, despite the socialite being married to the Rolling Stones rocker at the time.

The letter - which was dated September 30, 1973 - read: ''My dear Stan, I had the pleasure of meeting R. O'Neal while we were both guests in the home of rock-star Mick Jagger . . . and his charming wife, Bianca -- to whom, unless I am very much mistaken, young O'Neal was putting his stout wood in a most forceful manner.

''In fact, Stan, I would be less than candid if I failed to mention how I walked into the room one afternoon to find them going at it like a pair of maddened wart-hogs.''

Elsewhere in the letter, which Terry wrote to congratulate Stanley on casting Ryan in 'Barry Lyndon', the satirist detailed the ''tumultuous abandon'' of the pair's lovemaking.

The note - which is being published in a forthcoming issue of Man of the World magazine, who discovered it in the author's archive - added: ''O'Neal's coarse animal member [working] with such tumultuous abandon as to literally smash the darling girl against the sideboard at the very mo[ment] when yrs tly was pouring himself a short one! (sic)''

Bianca, now 70, previously admitted in 1977 that she had had ''an 'off-and-on' two-year fling with Ryan O'Neal'' while married to Mick, who she split from in 1978.