Ex-MP Luciana Berger has announced she has re-joined Labour after she quit the party four years ago due to concerns over anti-Semitism.
The former Liverpool Wavertree MP of almost a decade left the party with "no other choice" in 2019 and joined the Independent Group.
In a letter to Ms Berger published at the weekend, Mr Starmer again apologised and said: "That day will forever be a stain on Labour's history".
He added: "My test for change was whether those who were rightly appalled by how far we had fallen believe this is their party again.
"I know we've more to do but we're unrecognisable from the party that forced her out."
In her response, the ex-MP, who contested the last general election for the Liberal Democrats, said she was "pleased to be returning to my political home".
She added: "I am looking forward to rejoining the party and working with you to continue what you have started.
"It is time to replace this reckless and divisive government and ultimately make the difference our country so desperately deserves."
In her letter she said it was a "grim journey" from 2015 to 2019 "during which the party fell into the depths of the abyss under Jeremy Corbyn's reign".
She said it was "agonising" to be put in a position in which she had no other choice but to leave her "political home".
The equalities watchdog announced almost a fortnight ago the party had been taken out of special measures - after a 2-year anti-Semitism probe.
In October 2020 the Equality and Human Rights Commission (EHRC) found the party was responsible for unlawful acts of harassment and discrimination.
Mr Starmer said the findings of the EHRC investigation were "hard to read", adding that it was "a day of shame for the Labour Party".
Ms Berger's decision to rejoin the party also came as Labour's election war chest received a substantial boost from a former donor.
According to The Observer, Lord David Sainsbury, the former chairman of the Sainsbury's supermarket chain, has gifted the party £2million.
In a statement, he said: "I believe that Keir Starmer has the leadership skills and ability to deal with the economic difficulties we currently face.
"He also wants to reunite the country around an agenda of economic growth and social justice, which is one I strongly support.
"I am, therefore, helping the Labour party financially, as I don't think democratic elections should be decided by which party is able to raise the most money, and because I want to see Keir Starmer become Prime Minister as soon as possible."
A Labour Party spokesman said: "Donors are coming back to Labour because they can see we are a changed party that is serious about getting into government and building a fairer, greener, more dynamic Britain.
"We are very grateful for all support, large or small, as we gear up to fight the next general election.
"Thanks to Keir Starmer's leadership, the Labour Party is in a strong and positive financial position, having experienced significant growth in income throughout 2022, leaving us debt and deficit-free."
