Is it not the Red Ensign?
Regards
Luc
Edit : Hmmm no: Blue Ensign
No Luke, right first time.
NOW:
Red ensign - commercial/private vessels
Blue ensign - Royal Navy Fleet Auxilliary
White ensign - Royal Navy
This is a good website for the history:
http://www.sea-dreamer.com/page.asp?pagename=ensign
THE PAST:
The flags were all Royal Navy (until 1860's) to denote squadron colour.
I think it was
Red - Pacific Australia also any ships not assigned.
White - Home fleet and Atlantic
Blue - Mediterranean.
Stu