An offensive message written on the nail of Lindsay Lohan's middle finger might get her charged with contempt of court.

Lohan was sentenced yesterday with 90 days in jail by a Los Angeles judge. Inside the courtroom, she was being photographed and the cameras' lenses caught the "f**k u" written on her middle fingernail.

The troubled actress has denied that the message was aimed at Judge Marsha Revel who found her guilty of breaking probation from a 2007 drunk-driving charge.

"It had nothing to do w/court," Lohan wrote on Twitter.

"It's an airbrush design from a stencil."

In the meantime, New York City Public Defender Stacy Schneider told Fox News that if it was deemed that the defiant message was directed at the courts, then "it would be a separate charge for contempt".

"She could have an entire extra sentence heaped on top of her current one. If the judge were angry enough, it could run even consecutively," she was quoted as saying.

Ms Schneider adds that the fact Lohan even appeared in court with the explicit manicure was "the most outrageous thing" and that her lawyer should have done something to keep it from happening.

"It's up to her lawyer to catch things like that.

"A bra strap that shows or a boxer line that shows is pretty bad, but this tops the cake."

Los Angeles Public Defender Greg Apt told Fox News if Lohan was found guilty of contempt of court, there could be an additional three to five days behind the bars "for vulgarity".

While Lohan was sentenced to 90 days in jail, US media report the actress is likely to serve only 23 days behind bars because of prison overcrowding.