Rugby World Cup organisers, England Rugby and the City of London Police are working together to carry out a full investigation into suspicious activity around the ticketing market.
Detective chief superintendent Dave Clark said: "Anyone involved in these practices should not be surprised to receive a knock on their door before, during or after the tournament."
Rugby fans are being told to stick to official channels when purchasing tickets in order to avoid missing out on the tournament.
Around 50,000 tickets are still available for 27 matches at the tournament, including games involving France, Australia and New Zealand.
England Rugby managing director Stephen Brown told fans:
The discovery of these counterfeit tickets shows that there are unauthorised sellers and websites looking to exploit fans and we thank the City of London Police for their assistance in this matter. With tickets still available via all official sources, we would urge fans to only use these channels to ensure that they have the best possible Rugby World Cup experience.
An example of a counterfeit ticket:
And how a genuine ticket should appear: