Prince William jokingly told employees at BAE Systems in Barrow-in-Furness, Cumbria that they can "get back to work" after watching him unveil a plaque at the factory.
The occasion was to mark the opening of a new suite of offices where the next generation of Britain's nuclear submarines will be designed.
More top news
The note was left for paramedics attending an emergency call in Stoke-on-Trent on Sunday morning.
Monday will see a dull, damp and murky start, with temperatures reaching highs of 13C (55F).