Spokesman Neil Reading announced that the pair made the decision six months ago but had continued to live together in "constant discussion".
"The separation is entirely amicable and they fully intend to maintain their close friendship," a statement read.
The two - who met on the comedy circuit before marrying in Westminster, London - adopted daughter Billie in 1991.
The statement added: "Their priority is to commit to the future joint parenting of their daughter. They ask that the media respects the privacy of their family."
Vicar of Dibley star French moved the family to Cornwall in 2007, where it is thought she intended to retire with her husband.