EXCLUSIVE

Not only did Valerie Harper have the nerve to get cancer without telling the bigwigs behind her recent Broadway play ... she had the audacity to let it spread to her brain, causing her to forget her lines ... this according to the most tasteless lawsuit we've ever read.

Playwright Matthew Lombardo and the producers behind his Broadway production of "Looped" are suing Harper for $2 million, claiming she intentionally failed to notify them in 2012 that her cancer had returned with a vengeance ... all in a sinister plot to keep her job.

According to the lawsuit, Harper couldn't keep the ruse going for long -- Lombardo and co. claim her evil plan fell apart when she began to forget her lines and slur her speech during rehearsals.

Lombardo claims the director urged Harper to see a doctor, which she did -- and it was during that visit that she discovered her lung cancer had spread to her brain.

Harper was forced to pull out of the show due to her health issues -- no duh -- and according to the lawsuit, she selfishly left Lombardo scrambling to find a suitable replacement. At the time, Harper was given just months to live.

Lombardo claims the inconvenience cost him $500,000. He also wants an additional $1.5 million to punish Harper for her heinous misdeeds.