Only days after posters mysteriously appeared in Delhi and Ahmedabad criticising his forced exit from the BJP national executive, Joshi sent a brief communication to party president Nitin Gadkari.
"Sanjay Joshi has requested Nitin Gadkari to relieve him from the party. The request has been accepted," BJP spokesman Prakash Javadekar said in a brief statement here.
Javadekar refused to go into reasons for Joshi's sudden move but said the resignation was sent Friday.
An aide to the otherwise media-shy Joshi told IANS that the Rashtriya Swayamsevak Sangh (RSS) activist would not talk to journalists and would not give any interviews.
But a source quoted Joshi as telling some journalists at his residence here that he had decided to leave the BJP because of Modi.
A former lecturer in engineering, Joshi was seconded by the RSS to the BJP in 1988 when the party was growing on the strength of the Ayodhya movement.
He was later moved to Gujarat, where he once worked with Modi but played a major role when Keshubhai Patel, who is now at loggerheads with Modi, became the chief minister.
As his relations with Modi deteriorated, a sleaze CD apparently featuring Joshi forced him to leave the BJP in 2005. The CD was later found doctored.
Gadkari rehabilitated him, inducting him into the national executive and making him in charge of this year's Uttar Pradesh assembly election campaign.
This upset Modi, who stayed away from a BJP leadership meet in October last year and the Uttar Pradesh campaign this year.
Gadkari had to eventually ask Joshi to leave the national executive to placate the Gujarat chief minister and to ensure's his attendance in last month's BJP leadership meet in Mumbai.
BJP sources recalled that Modi had insisted before the May 24-25 Mumbai meet that Joshi should not only be asked to leave the national council but also the party.
The developments have caused divisions within the BJP and RSS, with one section critical of the way Joshi has been treated and another saying that Modi had to be accommodated in view of the coming Gujarat assembly elections.
A key party leader close to the RSS told IANS: "Perhaps the party wanted him (Modi) to feel at home in the party."
Another party leader said a lot of leaders were upset with Joshi after the poster episode.
A member of the national executive who did not wish to be named agreed that Joshi's fall was a clear sign that Modi was emerging as the party's most preferred prime ministerial candidate.
"People are awestruck with his charisma... He is definitely the next big thing in the party," the leader told IANS.
Some in the party are sympathetic to Joshi.
"It is bad the way he has been made a scapegoat to satisfy one leader (Modi)," a party leader from Delhi said on the condition of anonymity.
"When we say BJP is a party of workers, this is a case of cheating the workers," he added.
In Gujarat, Joshi enjoys the support of those still in the BJP but who are upset with Modi's style of functioning. Those who have left the party in the state also because of Modi are also considered close to Joshi.