The national flag of England, a constituent country of the United Kingdom, is derived from Saint George's Cross (heraldic blazon: Argent, a cross gules). The association of the red cross as an emblem of England can be traced back to the Late Middle Ages when it was gradually, increasingly, used alongside the Royal Banner. It became the only saint's flag permitted to be flown in public as part of the English Reformation and at a similar time became the pre-eminent maritime flag referred to as a white ensign. It was used as a component in the design of the Union Jack in 1606.

It has been widely used since the 1990s, specifically at national sporting events, especially during the campaigns of England's national football teams. Wikipedia