The Duke of Cambridge is to make his first appearance since it was revealed he would become a dad for the third time.
William will attend a policing conference in Oxford focusing on mental health, the day after Kensington Palace announced the Duchess of Cambridge is expecting a baby.
Kate, who is less than 12 weeks pregnant, was forced to pull out of a public engagement on Monday due to severe morning sickness.
The Duchess is suffering from hyperemesis gravidarum, as she did in her previous pregnancies, and is being cared for at Kensington Palace.
William, who stepped down from his job as an air ambulance pilot in the summer, has pledged to raise awareness of the mental health issues facing first responders.
He will give a speech at the National Mental Health and Policing Conference on Tuesday, before meeting professionals directly affected.
The event, part of an effort to create a national police strategy to address the issue, is being hosted by the College of Policing and the National Police Chiefs Council.