The 36-year-old has been dating English actress Egerton since 2012, after they met on the set of The Lovers in 2011.
Harnett, who has dated many high profile women including Scarlett Johanssen, Mischa Barton and Amanda Seyfried, is said to be ‘thrilled’ with the news.
Tamsin, 26, is expecting her first child with Josh
Tamsin, 26, made her name in films such as St Trinian’s, Keeping Mum and Driving Lessons, and has since starred in Chalet Girl and Grand Piano, amongst others.
The English actress, originally from Hampshire, has yet to speak out on social networks about her exciting news, appears to split her time between America and the UK since hitting the big time, but we wonder where her and her US beau will choose to settle with their bundle of joy?