This is why you don't touch a royal guard

A video that shows what happens when a hapless tourist crosses the line with a Buckingham Palace guard has gone viral online.

The clip shows a tourist marching alongside one of the Queen's Guard, aping the soldier's brisk pace.

However, he goes too far when he reaches out to rest a hand on the guard's shoulder.

The soldier wheels around and holds his bayonet at ready, calling out "Step back from the Queen's Guard!"

The tourist seems to hurriedly obey, as he is not seen for the remainder of the clip.

The video was uploaded to YouTube several days ago and has been viewed more than 800,000 times.