UEFA Champions League theme is an adaption of George Händel's royal coronation anthem

One of Handel's best-known pieces, Zadok the Priest has been sung prior to the anointing of the sovereign at the coronation of every British monarch since its composition and has become recognised as a British patriotic anthem. It was composed for the coronation of King George II in 1727.
The text is an adaptation from the biblical account of the anointing of Solomon.

And all the people rejoiced, and said:
God save the King! Long live the King!
May the King live for ever,
Amen, Alleluia.

These words have been used in every English/British coronation since that of King Edgar in 973.

Zadok the Priest is written for chorus and orchestra in the key of D major.
Here is a delightful performance of this piece by the Choir of Westminster Abbey:

In 1991, UEFA began re-branding the European Cup. This process resulted in the Champions League's anthem, as well as its "starball" logo.
The anthem was written by the English composer Tony Britten, adapted from Händel's mentioned anthem. In a newspaper interview, Britten commented:

"I had a commercials agent and they approached me to write something anthemic and because it was just after The Three Tenors at the World Cup in Italy so classical music was all the rage. Hooliganism was a major, major problem and UEFA wanted to take the game into a completely different area altogether. There's a rising string phase which I pinched from Handel and then I wrote my own tune. It has a kind of Handelian feel to it but I like to think it's not a total rip-off."

The piece was performed by London's Royal Philharmonic Orchestra and sung by the Academy of St. Martin in the Fields.

The lyrics are a mix of UEFA's three official languages: English, French and German. Full lyrics with the English translation can be found on LyricsTranslate.

Here is another less serious but touching performance of the anthem by the youth orchestra and choir from Brazil: