The decision was taken following a petition by AAP legislator Saurabh Bharadwaj after Lamba joined the Congress earlier this month. He sought her disqualification on grounds of defection.
The Speaker said Lamba, an elected member of the Delhi Assembly from Chandni Chowk, had been disqualified. "Consequently, the Chandni Chowk Assembly constituency has fallen vacant," the order said.
Earlier four more MLAs were disqualified on similar grounds
Reacting to this, Lamba said her journey with AAP has ended.
"My journey with AAP has ended now. I want to thank all those who supported me in my journey. I will continue my fight now with my public," she said in Hindi, adding the "democracy in AAP is ending with the dictatorship of one man".
On September 6, Lamba announced her exit from AAP and resigned from the primary membership of the Arvind Kejriwal-led Party on Twitter.
"The time has come to say "Good Bye" to AAP and to resign from the primary membership of the Party. The past six years journey was a great learning for me," Lamba said in a tweet while resigning.
The Chandni Chowk MLA, who has not been on the same page as the party on various issues since past several months, had later said she has joined the Congress on the same day after her meeting with Sonia Gandhi, the interim President of the grand old party.
( With inputs from IANS )