Colin Firth and his wife admit she used to bang the guy they're now accusing of stalking them. Stuff that's in Bridget Jones' diary!
The Oscar winner and his wife, Livia Giuggioli, a movie producer, released a statement saying she was "involved with" an Italian journalist named Marco Brancaccia ... who is also friends with the couple. Or, at least, he was.
Colin and Livia say her affair went down back in 2015, when she and Colin had temporarily separated.
The other man, Brancaccia, claims Livia intended to leave Colin for him -- and he has messages, photos and videos to prove it -- but she got cold feet.
As The Times first reported ... the Firths say Brancaccia is now stalking Livia, which he denies, and Italian polizia are investigating.