The Royal Standard pub in Wallingford, Oxfordshire has banned under-25s in the evening for "refusing to adhere to social distancing".
A statement on the pub's Facebook page said: "Please note that from 8pm tonight we will be implementing an over 25s-only rule in the evenings.
"This is because the last three nights have seen us attract unprecedented numbers of younger drinkers who are refusing to adhere to any social distancing rules which, in the current climate, is putting both our staff and our regular customers at risk.
"We apologise to those of you who are affected and we will review this decision periodically until we are in safer times but, until then, if you are under 25 and it is later than 8pm, you will be refused entry."
Landlord David Haines told the BBC: "In spite of how many times you ask them to sit in groups of six, it goes out the window. It was impossible to control. We've got a greater duty to our customers and our staff."