The WRU has made it clear that if the companies do not cease offering match tickets as part of unofficial hospitality packages, they could face legal action.
The letters sent out are part of a major operation being launched by the WRU to stamp out all unofficial hospitality around the Six Nations.
Supporters are advised to urgently check the providers of their hospitality as official packages can only be supplied by the WRU or its sole official off-site hospitality operator, Events International.
Any fans who have purchased unofficial hospitality packages for the RBS 6 Nations matches at the Millennium Stadium are being urged to contact the WRU as they face being refused entry to the games against England or Ireland.
The WRU's move to police the sales and distribution of its match tickets is to ensure money is kept within and reinvested in the national sport of Wales and for the first time ensures that WRU member clubs can distribute to an official and regulated off-site hospitality provider.
A number of WRU clubs have already participated in the WRU's scheme and sold tickets to Events International which is totally sanctioned by the union.
The WRU's approach will clear up any confusion for supporters who want to know that the hospitality package they buy is genuine and will ensure they get into the Millennium Stadium on matchday.
Head of Group Sales and Marketing, Craig Maxwell, said: "Enforcement teams will be in operation throughout the city and at turnstiles on matchday checking the tickets and passes to ensure they have come from official sources and will turn away ticket holders who have purchased through unofficial hospitality operators.
"By appointing an official supplier of off-site hospitality the WRU can control the trade and regulate the standards being offered to supporters.
"The RBS 6 Nations tournament is arguably the best rugby tournament in the world. It provides memorable experiences on matchday in and around Cardiff and the WRU is passionate about protecting the interests of the genuine fan."
All the member clubs of the WRU have been reminded of the ticket distribution conditions under which they receive their ticket allocation and have been requested to advise the WRU if they believe that they have distributed their tickets inadvertently or otherwise to unofficial operators. The WRU is now able to recognise tickets that have been the subject of unofficial hospitality.