The Duke of Cambridge carried out a visit to the Secret Intelligence Service, known as MI6, in London on Wednesday, according to the Court Circular.
Further details about Prince William's engagement have not been disclosed so far by Kensington Palace, but his outing came just hours before Russia's invasion of Ukraine.
It is understood the visit had been in the Duke's diary for a number of weeks.
READ: Kate Middleton reveals Prince William's concerns as she says she's 'broody'
WATCH: Duchess Kate reunites with Crown Princess Mary in Copenhagen
Last November, the future King, 39, also visited MI5 – the Security Service – at 12 Millbank.
And back in 2019, William spent three weeks quietly working with each of the UK's Intelligence Services – MI5, MI6 and GCHQ – working full-time alongside operatives and agents to get an understanding for their crucial roles.
Speaking at the end of the placement, the Duke said at the time: "Spending time inside our security and intelligence agencies, understanding more about the vital contribution they make to our national security, was a truly humbling experience.
"These agencies are full of people from everyday backgrounds doing the most extraordinary work to keep us safe. They work in secret, often not even able to tell their family and friends about the work they do or the stresses they face."
MORE: Duchess Kate reunites with Crown Princess Mary on day two of Copenhagen royal visit - best photos
MORE: Prince William and Kate's children just got offered an amazing opportunity
© Photo: Getty ImagesThe Queen visited MI5's headquarters in February 2020
"They are driven by an unrivalled patriotism and dedication to upholding the values of this country," he continued. "We all owe them deep gratitude for the difficult and dangerous work they do."
The Duke's latest visit came as his wife Kate was carrying out a two-day solo trip to Copenhagen with her Royal Foundation Centre for Early Childhood.
© Photo: Getty ImagesThe Duchess and Crown Princess Mary in Copenhagen on Wednesday
The Duchess travelled to Denmark to learn first-hand from academics and health professionals about their country's approach to the early years development of children that has made it a world leader.
On Wednesday, she was also reunited with Queen Margrethe and Crown Princess Mary at Christian IX's Palace.
Make sure you never miss a ROYAL story! Sign up to our newsletter to get all of our celebrity and royal news delivered directly to your inbox.
