LONDON—It looks like last call for a 400-year-old British rule requiring many pubs here to buy their beer from the large companies that own them.

Shares in Britain’s largest pub companies fell sharply Wednesday after lawmakers voted the previous evening to allow “tenanted” pubs to buy their beer on the open market. The change would let publicans opt out of the “beer tie,” a centuries-old system that critics say force publicans to overpay for beer.

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