In her letter to the Irish guards, the Princess of Wales signs herself Colonel Catherine to indicate her rank as Colonel of the Regiment.

British Army regiments typically have an honorary colonel, often a member of the royal family or a prominent retired military officer with connections to the regiment.

The Princes of Wales was appointed Colonel of the Regiment of the Irish Guards in December 2022, succeeding her husband the Prince of Wales, who had been appointed Colonel of the Regiment in February 2011.

While William was on active service and on operational duties with his unit, he was referred to by rank and surname as Lieutenant Wales, in standard military practice. He is now Colonel-in-Chief of the Army Air Corps.

But the Princess of Wales’s title of Colonel is purely honorific, allowing her a degree of informality when referring to herself as Colonel Catherine.

A palace source described the letter to the regiment as “a really significant moment for the Irish Guards”, with its tone indicating a reciprocal comradeship and respect between the troops and their absent Colonel.