CAIRO, Jan 1 (Reuters) - An Egyptian satirist who made fun of President Mohamed Mursi on television has been accused of undermining his standing and will be investigated by prosecutors, a judicial source said on Tuesday.

Bassem Youssef's case will increase worries about freedom of speech in the post-Hosni Mubarak era, especially when the country's new constitution includes provisions criticised by rights activists for, among other things, forbidding insults.

Youssef rose to fame following the uprising that swept Mubarak from power in February 2011 with a satirical online programme that was compared with Jon Stewart's Daily Show.

He has since had his own show on Egyptian television and mocked Mursi's repeated use of the word "love" in his speeches by starting one of his programmes with a love song, holding a red pillow with the president's face printed on it.

The prosecutor general ordered an investigation into a formal complaint against Youssef by an Islamist lawyer. The complaint accuses him of "insulting" Mursi, an Islamist backed by the Muslim Brotherhood, and "undermining his standing".

Human rights activists say it is the latest in a series of criminal defamation cases that bode ill for free speech as Egypt reshapes its institutions after Mubarak was toppled.

"The greatest threat to freedom of expression over the last four months has been this rise in criminal defamation cases, whether it is on charges of defaming the president or the judiciary," said Heba Morayef, Egypt director of the New York-based Human Rights Watch.

"The problem is now is we are likely to see an increase in this because criminal defamation is now embedded in the constitution," she said.

Rivals accuse Mursi, who won Egypt's first freely contested leadership election in June, of polarising society by foisting a divisive, Islamist-leaning constitution on the country and using the autocratic ways of his deposed predecessor. (Writing by Tom Perry; Editing by Alison Williams)