'Dancing With the Stars' -- Threatens Dance Pros ... Don't Tango With the Competition

"Dancing With the Stars" is drawing a line in the parquet floor with some of its famous pros -- warning them to stay away from an upstart live dance show, or risk a tango ... in court.

Our 'DWTS' sources tell us producers fired off warning letters to several dancing pros right after they sued a touring show called "Ballroom With A Twist." The letter informs the dancers they could be violating their "DWTS" contract if they do any side gigs with the rival.

'Dancing' vets Peta Murgatroyd, Tony Dovolani and Val Chmerkovskiy have all appeared on promotions for 'Ballroom' ... but the show hasn't started touring yet. We're told they've now been told to bail, or face potential legal action.

In other words ... they own you, jive turkeys.