I trawled through the Cunard website and could find nothing about it one way or another.

They may (indeed should) ensure in a direct communication to a booked passenger what their policy actually is.

My one cruise experience with Fred Olsen lines permitted it but it was still taken from me by security at embarkation at Southampton and I was told to collect it from the purser in time for the formal cocktail party with the Captain. The security guy told me that if I had been sailing with P&O it would not have been allowed.

I then had it for the rest of the evening and nobody turned a hair when they saw me wearing it. The person on the Purser's desk seemed puzzled as to what to do with it when I took it back. I was even allowed to get it back before we docked in Rosyth.

So no consistency by the cruise companies - clearly if someone, like Jimmy, chooses not to bring one to wear then that's fine as no policy is being breached but if someone wants to bring one they need clear guidance in advance. If the other guy photographed with Jimmy had been wearing one then we would known it's OK with Cunard but because he wasn't was why I asked the question - one which I still do not know the answer to.