Although the party refused to comment on the development calling it an "internal matter", sources said the Chandni Chowk MLA was asked to quit the post.
Lamba came under fire after giving a statement that Rai had been "relieved" of the portfolio so that the AAP government's premium bus scheme could be investigated by the anti-corruption branch in a free and fair manner. The opposition Bharatiya Janata Party had approached the ACB demanding a probe into this matter.
The ruling AAP had maintained that Rai was divested of the transport portfolio on his request that he had undergone a surgery.
Reacting to the development, Lamba, who is also a parliamentary secretary, said she was a disciplined worker and respected the party's decision.
"If I have inadvertently made any mistake then I will surely do penance for it so that my party, which has been fighting corruption, does not get harmed because of me," she wrote on Twitter.
Party sources said the AAP MLA had been warned several times for her statements and comments on social media, which were not in sync with the party.
"Lamba had been given instructions not to talk or post anything if she was not aware of the party line. In the past 7-8 months, she had been warned several times.
"In this matter when Chief Minister Arvind Kejriwal was already speaking, there was no need for her to make a statement which was incorrect," a senior party leader said.
AAP's Delhi unit convenor Dilip Pandey said the Transport Minister was relieved of the post "purely" on health grounds and one should not "connect wrong dots".
"His health was really bad and the party is very concerned about him. He still spends around 95 per cent of his time in hospitals. The organisation decided to relieve him of the portfolio which consumes a lot of time and energy. This could allow him to concentrate on his health," he said.
On June 14, Rai was relieved of the charge of the Transport department as sought by him on health grounds, even as the Opposition BJP and Congress termed it a fallout of an ACB probe into the AAP government's app-based premium bus service scheme.
Rai refuted the allegations saying that he quit solely on health grounds. He also "voluntarily" presented himself before ACB and claimed that the probe agency was "totally clueless" about how to substantiate the graft charges in the scheme.
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