"I do not deem it morally and intellectually appropriate to continue to serve on the party's national executive as long as you are the president," Jethmalani said in a terse single-sentence letter to Gadkari.
Mahesh's decision came nearly a fortnight after his father and BJP's Rajya Sabha MP Ram Jethmalani asked Gadkari to quit the post and not to seek a second term in office in the wake of allegations against him.
Gadkari is in the line of Opposition fire following media reports about dubious funding of his company Purti Sugar and Power.
Party leaders have maintained that Gadkari himself has offered to face any probe in the allegations but murmurs within are said to have forced him to cancel electioneering in Himachal Pradesh last week.
Talking to reporters in Delhi, Mahesh said he has not got a clear answer on the allegations against Gadkari. "Since my personal conscience was not satisfied I have chosen to resign from the national executive. I will continue to serve the party," he said.
Asked if he had written to Gadkari seeking his clarifications on the issue relating to his company, Mahesh said 'no'.
"But based on his defences that are in public domain, I have made my personal analysis." To a question whether the allegations against Gadkari has affected the party's image, he said, "Yes. I feel that it's affecting the party's and we are delaying it (a decision). I would like to say this party leadership, that this issue should be resolved as soon as possible."