Those terms and conditions stipulate any unwanted tickets must only be sold on only via the RFU's official online ticket exchange, as well as warning anyone found in possession of a ticket obtained though unauthorised means will be denied entry to this weekend's match and risk being barred from attending future games at Twickenham.
With Cheltenham Festival also taking place this week, Irish sports fans are expected to travel to the UK in their thousands ahead of what could be their country's third Grand Slam since 1948 following their nail-biting triumph in Wales nine years ago.
An RFU spokeswoman told Telegraph Sport: "The England v Ireland match at Twickenham is sold out, and has been for a long time. There is huge interest in the match.
"Our stance on the secondary market is very clear – Twickenham tickets cannot be sold on secondary websites. It is a clear breach of our ticketing terms and conditions, and we are constantly monitoring the marketplace.
"Our message to rugby fans is clear – don't risk your money by purchasing tickets from a secondary site because if you do, there is a good chance you won't be able to get in.
"As ever, we want tickets to go to rugby fans who want to come to the match – and not to people who want to trade them as a commodity."
