Among 18 to 24-year-olds, only 30% say the monarchy is "good for Britain", compared with 77% among the over-65s.
The survey of more than 2,000 adults in Britain comes as the first anniversary approaches of the death of Queen Elizabeth II.
The pollsters found that overall, 62% want to keep the monarchy.
But they report a "remarkable difference between generations", with younger people much less supportive on remaining a monarchy and more sceptical about the Royal Family representing good value for money.
For King Charles, as he approaches his first year on the throne, 59% of people thought he was "personally doing a good job".
The pollsters say there has been a broadly consistent picture of "overall positivity towards the monarchy", but there is also a sizeable and rising minority who are opposed.
While they head up various charities and 'good work' as such, their value to the UK is really now only in tourism. People like travelling to the UK to see some of its history given that it is still 'current', so there are palaces to see, ceremonial stuff like changing of the guards. If we didn't have that, it would be one less reason to even travel to Britain I guess, and tourism is a large part of our GDP (over £130 billion). Reasons are, I imagine, diminishing...
When the Queen was alive, there was a very subtle soft power over Parliament because some Prime Ministers absolutely did not want to sit in front of her during their weekly audiences describing what a shambles they were making of the job and how they were failing people. I might be wrong, but I don't feel Charles has that same presence, so that has diminished too.