The couple's representatives announced that they briefly split three years ago but reunited shortly, after which Brancaccia harassed the Italian producer.
The news was out in the open as they were involved in a police investigation, The Times reported.
"A few years ago Colin and Livia privately made the decision to separate. During that time Livia briefly became involved with former friend Mr Brancaccia. The Firths have since reunited.
"Subsequently, Mr Brancaccia carried out a frightening campaign of harassment over several months, much of which is documented. For obvious reasons, the Firths have never had any desire to make this matter public," the statement from Firth and Guiggioli read.
The couple married in 1997 and have two children Luca, 16, and Matteo, 14.