white ensign 
noun
A white flag carrying a St George’s cross with the Union Jack in the top corner next to the flagstaff, flown by the Royal and most Commonwealth navies and the Royal Yacht Squadron: take her to sea under the white ensign...
- He was hauling it down and hoisting the white ensign of the British Navy.
- At that time, the white ensign, like the others, was a plain flag with a St George's cross in the canton.
- Team leader WO Richards said the visit was a good opportunity to show the white ensign in a city that doesn't get to see many ships sail by - apart from the annual Henley on Todd River regatta.