Tours of Big Pit, one of Amgueddfa Cymru's seven museums, are led by former miners and explore original mine shafts.
A sign outside the entrance now states: "Due to financial pressures and funding cuts, from 5 April we will be charging for the underground tour."
But Delyth Jewell MS, chair of the Senedd's culture and sports committee, said the tours are "a fundamental part of our national collections, they are part of our national story, and that shouldn't be limiting if people can't afford to access them".
The report says the Welsh government has "not fulfilled its responsibility in funding museums adequately", citing a 17% cut in real terms to funding for culture and sport in Wales in the past decade.
"We appreciate that it is expensive for Amgueddfa Cymru to provide access to the underground workings at Big Pit," it says.
"However, we think this is an argument for the Welsh government to provide an adequate level of funding, not for Amgueddfa Cymru to charge for access."