You may think this is the flag of England.
Actually the story is a little different.
At the beginning it was the flag of the ancient Republic of Genoa and its navy. It is said the cross was originally used by the byzantine troops stationed in Genoa. In 1190 the King of England asked the permission to expose the same flag on the english ships in oder to get the protection of the genoan fleet against the pirates in the mediterranean sea, paying an annual tribute to the Republic. Following this agreement the flag became the symbol of London, England and the royal navy.
The same flag was officially given in year 1066 by the Pope Alexander II to the city of Milan that was rebelling against the emperor (see Lombard League).
It was used by the Lombards who got part to the first crusades and it was widely used by all the crusaders as part of the common uniform. St. George was considered the protector of the crusaders, following a legend of the saint appearing before the battle of Antiochia wearing an armor with the red cross. This legend of course resambles the famous vision "in hoc signo vinces" (you will win with this symbol) of the roman emperor Costantinus and the following adoption of the cross by the roman armies. It is also said that St. George actually is the christianized version of ancient German and/or celtic warrior gods.
In case somebody asks why the Internazionale Football Club, one of the team of the city of Milan, uses this shirt when they play outside the city. Recently the team played against the Fenerbace from Turkey and won. Now it seems some people in Turkey got offended by the shirt because they say it is anti-islamic and are asking the UEFA commettee to give victory to Fenerbace as punishment.