Flag associated with Isaac Komnenos, the governor of Cyprus. 
According to English heraldic tradition, when  Richard I Lionheart conquered the island of Cyprus in 1192, he took a banner of this description from Isaac Komnenos and upon 
his return to England  bestowed Isaac's Arms to the city of Portsmouth. This heraldic tradition can be traced with certainty to the 17th century, and its historicity is questionable.

"In 1194 it was King Richard I, Richard the Lionheart, who granted the Town of Portsmouth its first charter. The star and crescent symbol is also present on the seal of William de Longchamp, Bishop of Ely, who as Lord Chancellor, was involved in the in the granting of Portsmouth's charter. It is believed that the Town of Portsmouth adapted the symbol to use as its coat of arms in tribute to King Richard, for his patronage in granting Town status. The Star and Crescent symbol is known to have been used by Mayors of Portsmouth from at least the seventeenth century and probably earlier from the middle ages." (welcometoportsmouth.co.uk)
"Richard's first great seal showed on either side of his head a star with six wavy rays (known as an estoile) above a crescent moon. On some specimens of his first Great Seal an eight-pointed star was used. It is not known for certain whether Richard adopted this device as a result of going on the Crusades to Palestine in 1191, or whether it was a punning reference to the star called Regulus in the constellation of Leo, which is commonly known a 'Cor Leonis', or 'Heart of the Lion'" (www.ngw.nl/heraldrywiki/)Azure, an estoile above a crescent moon Or.
Transferred from en.wikipedia to Commons.br/&gt;(Original text : self-made)
Dragases at en.wikipedia