Knightley, who is now engaged to James Righton, dated model/actor Dornan for two years from 2003 to 2005, and he went on to strike up a romance with Warner, who records under the name Slow Moving Millie.
The Pirates of the Caribbean star revealed earlier this year (12) she is set to tie the knot with Klaxons rocker Righton, and now Dornan has announced wedding plans of his own.
The Once Upon a Time actor will exchange vows with Warner at a country house in Somerset, England. The date of the wedding has not yet been revealed, but it is believed to be soon as the couple has published its wedding banns, a formal announcement of the intention to marry.
Warner was previously engaged to Hollywood actor Colin Farrell.
They were rumoured to have secretly wed on a beach in Tahiti, French Polynesia in 2001 when Warner was just 19, but they split several months later and last year (11) the singer revealed the ceremony was not legally binding.