English: On 1 January 1801, the United Kingdom of Great Britain merged with the Kingdom of Ireland to become the United Kingdom of Great Britain and Ireland. In preparation for this, on 5 November 1800 at St James's Palace, the Privy Council of George III, King of Great Britain and King of Ireland, approved draught illustrations for the heraldry of the new kingdom prepared by the Garter Principal King of Arms (Isaac Heard) and Richmond Herald (Ralph Bigland). The 5 November 1800 Order in Council prepared a royal proclamation to be issued on 1 January 1801, announcing the new state's "royal style and titles" and "also the ensigns armorial, flags and banners thereof"; the Garter and Richmond heralds supplied "divers drafts" of these for the king's approval. The relevant parts of the Order in Council read:

And with respect to the ensigns armorial of the said united kingdom, the lords of the committee do agree to lay before your Majesty the annexed draft marked (A.) proposed for the bearing of arms for the said united kingdom, in order to receive your Majesty's pleasure thereon, both as to what relates to your Majesty's armorial bearings, and also to the armorial bearings of his royal highness the prince of Wales and the other branches of your Majesty's royal family with the several distinctions already assigned to them …

The committee are further of opinion, that the union flag should be altered according to the draft thereof marked (C.) which is herewith humbly submitted to your Majesty, and in which the cross of St. George is conjoined with the cross of St. Andrew and St. Patrick.

That the standard be the arms of the united kingdom, according to the draft marked (B.)

The royal proclamation of 1 January 1801 blazoned the new union flag as:

Azure, the Crosses Saltires of St. Andrew and St. Patrick quarterly per saltire, counterchanged argent and gules; the latter fimbriated of the second, surmounted by the Cross of St. George of the third, fimbriated as the Saltire.

The design has remained the flag of the United Kingdom and the British national flag ever since. The differing interpretations of the blazon, especially the phrase "fimbriated as the Saltire", have led to various flags with crosses and fimbriations of different proportions. The overall proportions of the flag itself also vary: according to the Garter King of Arms and the Earl Marshal, the official proportions for flying the flag on land are 5:3 (see: File:Flag of the United Kingdom (3-5).svg, while the proper proportions of the flag flown at sea are 2:1 (see: File:Flag of the United Kingdom.svg. (Union flags used as military colours also exist in other proportions; generally, British Army regimental colours since 1900 are in proportions 5:4.)