New York, Jul 17 (IANS): US prosecutors have made a plea for leniency for a former Indian-American executive of McKinsey & Co whose testimony helped convict Wall Street giants Raj Rajaratnam and Rajat Gupta on insider trading charges.

Anil Kumar's cooperation was "nothing short of extraordinary," federal prosecutors said Monday, in a letter to Judge Denny Chin three days before Kumar, 53, is expected to be sentenced Thursday in Manhattan federal court.

Kumar, who chose to plead guilty after being charged criminally, helped prosecutors and the Federal Bureau of Investigation in their wide-ranging probe of illicit trading on Wall Street, prosecutors said.

"From the first day of Kumar's cooperation through the present, he's been one of the best and most important cooperating witnesses that the (prosecutors) working on the Rajaratnam investigation have worked with in securities fraud cases," the letter said.

Kumar was essential in helping the government improve its case and convict Rajaratnam and Gupta, "two of the most important securities fraud trials in history," they said.

Kumar's testimony at the successive trials of the two men was particularly convincing because he had opted to turn against one person who had been his friend for over 25 years, and the other who had been his mentor at McKinsey, prosecutors said.

Galleon-group hedge fund founder Rajaratnam is serving an 11-year prison term after being convicted last year. Gupta, a former board member at Goldman Sachs who headed McKinsey & Co for nine years, was convicted in June. He faces up to 25 years in prison when he is sentenced in October.

As part of a plea deal with prosecutors, Kumar pleaded guilty in January 2010 to one count of conspiracy to commit securities fraud and one count of securities fraud tied to feeding Rajaratnam secret stock tips.