Cecelia Ahern has revealed she is expecting her third child with her husband David Keoghan.

The best-selling author and her husband of nine years attended the first day of Royal Ascot in the United Kingdom on Tuesday, and Ahern shared a series of photos on Instagram which shows off her baby bump.

In the photos, Keoghan cradles and kisses her baby bump while they both smile widely.

The couple are already parents to nine-year-old daughter Robin and six-year-old son Sonny.

The PS, I Love You's writer's latest book Roar is being adapted into a television series.

Ahern's titles have sold more than 25 million copies worldwide in nearly 50 countries.