It is a scam which the IRB originally sounded a warning about last year, but the letter to Nantymoel shows attempts are still being made to dupe rugby club members.
The IRB has alerted all unions of the scam and pointed out it does not operate and has never operated any form of sweepstake.
Nantymoel alerted the WRU through their Board member Aurwel Morgan who is now sounding a warning to all other clubs.
He said: "I am extremely grateful to Nantymoel for bringing this clear scam to our attention. It is vitally important that all our clubs and club members remain vigilant and do not fall into the trap of believing this appeal.
"The offer was made in a formal letter and it certainly appears to give the impression of being a valid document.
"The truth is it is a cynical attempt to procure money and the fact that Nantymoel received this letter recently means this scam is still active."