Police were called when a driver saw what they thought was a body wrapped in carpet - only to discover the figure was a mannequin of Prince Charming.
Cambridgeshire Police said the sighting of a pair of feet poking from the roll of carpet had set "alarm bells naturally ringing" for the witness and officers alike.
But once police had tracked down the driver - and ultimately the prince - they found out that it was a life-size figure being taken to a themed birthday party.
The motorist was given "advice... to avoid such circumstances occurring again", the force said.
Writing on social media, police said: "When we received a call on Saturday lunchtime to reports a vehicle was travelling along the M11 with a pair of feet poking out the end of a rolled up piece of carpet, alarm bells were naturally ringing for the informant and our control room.
"We managed to track the vehicle down in Angel Drove, Ely, where our officers discovered there was indeed a pair of feet... belonging to a mannequin dressed as Prince Charming who was on his way to a themed birthday!
"Advice was given to the driver to avoid such circumstances occurring again! Thankfully this was a false alarm."