The oldest, continuously used national flag is that of Denmark. The current design of a white Scandinavian cross on a red back ground was adopted in 1625 and its square shape in 1748. In Denmark it is known as the 'Dannebrog' or 'Danish cloth'.
Although Denmark was never part of the Roman Empire, similar designs were used by the Empire to represent provinces, as the white cross is symbolic of Christianity. The Scandinavian cross has its horizontal stripe slightly to the left hand side of the square as you face it.
Although there is no prescribed definition of what constitutes “continuous" use, the Danish flag was certainly in use in the 1370s, as the Gelre Armorial by Claes Heinenzoon (or Heynen 1345-1414) shows. It was also certainly used in the naval battles during the war against Sweden in the 1560s, as shown in a watercolour in Rudolf Dewenter’s Bericht von Pulver und Feuerwerken from 1585. In his War Articles, promulgated on 8 May 1625, King Christian IV issued the first known regulations for flying the flag and Colours of Command in the Navy in Denmark.
It is often claimed that the Scottish Saltire can lay claim to a longer continuous use as a national flag. According to the Flag Institute, evidence for both the Saltire and the Dannebrog is fragmentary yet their vexillologists believe that the balance of evidence points to Denmark as being older: although St Andrew was named as the country's patron saint in the 14th century, there appears to be no direct evidence that the Saltire was always carried on a blue flag from that date. Flags captured at the battle of Flodden in 1513 bore a saltire, but they were not white on blue. A bill for the manufacture of flags from the same year mentions a red St Andrews flag, not a blue one. The flags captured in 1649–50 by Cromwell after the battles of Dunbar and Worcester include ones with black or red saltires in addition to white ones. Yet a white saltire on a blue flag was thought of as sufficiently emblematic of Scotland for it to be included in the Union Jack in 1606. It would therefore appear that the saltire, and the saltire alone, that was the symbol of Scotland – the colours of the flag were not of the first importance, particularly in the first half of the 16th century. This would, according to the Flag Institute, bar it from being defined as “continuous" use of the modern flag.