The union jack can be used as a silent alarm due to its design, with the upper red diagonals being biased towards the hoist, and the lower ones towards the fly.
Historically, if a ship was boarded by an enemy who were hoping to infiltrate a British harbour, the crew could fly the jack upside-down. The enemy would not be aware of the difference, yet the coastguard would immediately notice, and alert the appropriate authorities.
According to Roger Helmer, MEP, it is now an internationally recognized distress signal.
"Not only does it show great disrespect to a nation to fly its flag upside down, but an upside-down Union Jack is an internationally-recognised distress symbol."
His comments came since the flag is regularly found flying upside down at the European Parliament.