Banner of arms of London's Metropolitan Police: Azure, a portcullis chained within a double tressure flory counterflory argent ("a blue flag with a white portcullis with chains inside a white double border with alternating fleurs-de-lys"). The "double tressure flory counterflory", similar to that on the royal arms of Scotland, is a reference to the police headquarters, Scotland Yard.
This file was derived from:

 Royal Banner of Scotland (4-5).svg
Badge of the Portcullis Pursuivant.svg
Own work based on: Royal Banner of Scotland (4-5).svg  and Badge of the Portcullis Pursuivant.svg