San Jose (California)
I will be traveling to various sites throughout Italy and France and even in London. My friend tells me most museums and tourist places do not allow me to carry my backpack. This is where I will hold my wife's jacket, camera, etc... Can anyone verify if that is accurate?
This is true, unfortunately - it's a security measure. Most places will allow you to take a small bag, but not anything as big as a day pack.
Museums / galleries will usually have somewhere secure you can leave it at the entrance, though, often for a small fee. Lots of museums in London (such as the Victoria & Albert Museum and the National Gallery) offer this for free, providing the bag isn't too big (as in, a day pack, not a 3-month backpack).
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I hope you are getting excited about your European adventure! When I lived in London I always carried around a bag that held my belongings. When you walk into museums they look through it, but I did not see anyone having issues with their bags.
Check out my blog where I have shared some of my experiences in those countries: http://theroadtakentravels.com
It depends on the museum. But yes, mostly bigger museums don't let you take everything inside. You leave you backpack in a locker but you can take some small things that you need with you.
Your friend is right! Most national museums in Italy don't allow backbacks or large bags inside, so you'll need to check it when you enter.