10:00 PM PST -- A source connected with Usher tells TMZ ... this is a frivolous lawsuit and all the allegations made by the former nanny are completely false.
Usher has secretly been stiffing the nanny ... financially ... this according to a new lawsuit obtained by TMZ.
Cecilia Duncan is suing the singer, claiming Usher hired her as a part-time nanny back in December 2010 and she worked her ass off caring for his two sons (aged 4 and 5) often exceeding 40 hours per week.
According to the lawsuit, Cecilia felt like she was being overworked without being paid overtime ... and expressed her concerns to Usher in 2011 that she was being shafted.
Cecilia says Usher ignored her complaints for more than a year ... and finally fired her ass last September without paying her overtime money.
She's now suing for wrongful termination, failure to pay wages, and more -- demanding unspecified damages. Sources close to the lawsuit tell us she wants in the six-figure range.