On April 12, 1606, The Union Flag (or Jack) was officially adopted as the standard of England, Wales, and Scotland. Though the Treaty of Union would not be signed until 1707, the three kingdoms were united through having a common ruler, James I of England/VI of Scotland. St. George's cross was joined by that of St. Andrew's. Wales was not explicitly represented on the Union Flag, as it had already legally incorporated into England.
The Union Flag makes an appearance in Walter Scott's "Waverley":
"...In about two hours' time, the party were near the Castle of Stirling, over whose battlements the union flag was brightened as it waved in the evening sun. To shorten his journey, or perhaps to display his importance, and insult the English garrison, Balmawhapple, inclining to the right, took his route through the royal park, which reaches to and surrounds the rock upon which the fortress is situated..."