red ensign

'The Red Ensign or "Red Duster" is a flag that originated in the early 17th century as an English ensign flown by the Royal Navy and later specifically by British Merchant Seamen (The British Merchant Navy). ... It is currently used as the civil ensign of the United Kingdom.' [1]

The United Kingdom version has a red flag with the Union Jack in the upper left.

The red ensigns of other countries have some variation on this.

See also