Flags of the English Interregnum

There were a variety of flags flown by ships of the Commonwealth during the Interregnum of 1649–1660. At sea, royalist ships continued to fly the Union Jack of 1606, while on 22 February 1649 the Council of State decided to send the parliamentary navy an order (signed by Oliver Cromwell on 23 February) that "the ships at sea in service of the State shall onely beare the red Crosse in a white flag" (viz., the flag of England).

Source: Wikipedia — Flags of the English Interregnum (CC BY-SA 4.0)