Hallmarking dates back to the 14th century when Edward I of England passed a law requiring any item made of silver, which was offered for sale to be at least of equal quality as that of the coin of the realm (silver currency). The four wardens of the Goldsmiths’ Company were tasked with visiting workshops in the City of London to assay (test) silver articles. If these articles were found to be below standard they were originally forfeit to the king, but if they passed, each article received the king's mark of authentication which was the mark of a leopard's head. By 1478, there were several hundred workshops and merchants manufacturing silver articles in the City of London. It was not possible for the wardens to visit them all so the merchants were ordered to bring their items to Goldsmiths’ Hall for testing and marking and a permanent assay office was established in the building. This is the origin of the term hallmark – struck with the king's mark at Goldsmiths’ Hall.

In 1544 the Goldsmith's Company adopted the king's mark as their town mark and the mark of the leopard's head is now internationally recognized as the mark of this assay office.
From the passage provided, extract when hallmarking was first observed in England.
Hallmarking was first observed in England in the 14th century.