Ms Abbott was suspended from the party in July last year.
Diane Abbott has been readmitted to the Labour Party following an internal investigation, the veteran MP has said.
Ms Abbott was suspended from the party in July last year after repeating comments about racism for which she had previously apologised.
Fellow Labour MP Joani Reid was also readmitted to the party on Thursday evening.
She had “voluntarily” suspended herself in March this year following the arrest of her husband on suspicion of spying for China.
In a statement on social media Ms Abbott said she had been “issued with a formal warning” over a separate tweet in which she referred to the Israel Defence Force as the “Jewish Defence Force”.
Apologising for the tweet, she said she had “used wording that conflates the actions of the state of Israel with the Jewish community” and was “deeply sorry for any hurt caused by that conflation”.
She added: “I have been and remain a lifelong anti-racist and remain deeply committed to opposing racism in all its forms.”
