Either name is acceptable.

In 1606, King James VI and I ordered all English and Scottish ships – warships and merchant vessels – to fly the flag of his new realm of Great Britain from their main masthead.

This flag was described initially as the ‘British flag’ or ‘flag of Britain’.

The term ‘Union’ first appears in 1625, but what of ‘jack’?

Jack can have a diminutive sense, denoting something smaller or slighter than normal.

Thus, it was used to describe a small flag flown by ships of this period from a flagstaff rigged on the bowsprit.

By 1627, it appears that a small version of the Union Flag – described thereafter as the ‘Jack’, ‘Jack flag’ or ‘King’s Jack’ – commonly flew in this position.

And by 1674, the Admiralty acknowledged two names: the formal ‘His Majesty’s Jack’ and the informal ‘Union Jack’.

During the following century, staysails replaced the bowsprit flagstaff, and the main distinguishing flag of the Royal Navy became the Ensign.

So naval vessels began to fly the ‘Union Jack’ only when in harbour and from a specially rigged ‘jackstaff’ in the bows.

The term ‘jack’ thus predates the ‘jackstaff’ by over 150 years.

Neither is ‘Union Jack’ a name specific to a Union Flag flown in the bows of a warship.

The Admiralty used both names interchangeably from the seventeenth century onwards, irrespective of use.

An Admiralty Circular of 1902 pronounced that either name was officially valid.

And in 1908 Parliament approved this verdict, confirming that ‘The Union Jack should be regarded as the National flag’.