Licensees face stiff penalties if they let DJs play unlicensed tunes via laptops in their venues.
This could include fines, a criminal record or losing the right to play background music at all, under the new licensing regime for DJs.
Anyone who uses a laptop to broadcast copyrighted music in bars — for karaoke, for example — must now hold a new Professional Dubbing (ProDub) licence.
The licence, from royalty collection agencies MCPS and PPL, gives the right to copy owned music to laptops to be used for entertainment. The basic cost for the first year is £250 to copy up to 5,000 tracks.
MCPS is writing to bars asking them to check if their DJs have the new licence.
If they don't operators risk breaking competition law, which could result in their existing PRS or PPL licences being revoked, plus fines and court appearances.
"We are keen to educate venue owners," said MCPS business development director Neil Jones. "The primary responsibility does lie with the DJs, but the venue owners should be aware of the potential implications."
Operators can check if their DJs have a ProDub licence by visiting www.produb.co.uk. MCPS urged licensees to report unlicensed DJs to info@produb.co.uk.
Paul Andrew, licensee of the Showman in Exeter, said: "Hopefully this will stop many illegal operators continuing to trade with illegally formatted and downloaded music."
But he added: "I do not think it fair that the venue operator could be punished when this ProDub licence has yet to be properly publicised, and the implications for many are unknown."