Hanson says a huge millennial faux pas is asking who is else coming when invited to an event.
"It should not matter who else is attending."
Niggly dietary requirements
The etiquette expert claims millennials try to "outdo" each other with their particular dietary requirements.
"Everyone has an allergy or dietary requirement, each guest trying to outdo the other for personal attention and concern from the host."
Poor time keeping
Hanson says millennials have no concept of being politely "late".
"'Come for 7.30,' you tell them. And then bang on the dot, or worse still, at 7.20, they present themselves at your door."
Forever connected
The etiquette expert says there's nothing more undignified than when guests ask for the Wifi password on arrival.
"Telephones should be switched off, onto silent and out of mind for the duration. Your full attention should be on your real life friends, not the digital ones."
Bad manners
Hanson says following attending a dinner party or lunch, guests need to make sure a thank you letter is in the mail no later than 36 hours after the event ends.
Clock-watching
According to Hanson, there's nothing ruder than making other plans for later on in the night.
"Some guests keep checking their watches. Why? Because they are off into town for drinks with other friends. They're packing in the social engagements that evening: they're so in demand (in their heads).
"If you accept a dinner invitation, snowflakes, it's one engagement per night."
Worst of all: bailing
Hanson says the rudest thing millennials are known for is not even turning up at all.
"Weeks ago it was fine for them to commit, but right at the last minute they have a bout of 'can't be bothered-itis' and announce their great aunt has just died, or that their flatmate has fallen down the stairs."
