The new Labour MP for Batley and Spen was greeted by MPs on both sides of the House as she took a seat on Wednesday morning, but "divisive" material used during her by-election campaign faced criticism from the Government.
Speaking at the start of Prime Minister's Questions, Sir Keir said “Will members opposite forgive me if I just turn round to look at the new member for Batley and Spen as she sits there on these benches beneath the plaque to Jo Cox, her sister.
"And that’s a special and emotional moment for all of us on these benches and I think for everybody across this House.
“It takes incredible courage and bravery to stand in that constituency and to sit on these benches beneath that plaque.”
In the earlier women and equalities questions, Conservative former minister Theresa Villiers had condemned a “divisive” leaflet used during Ms Leadbeater’s campaign, which she said was “designed to stoke up” tensions.
She said: “During the Batley by-election Labour circulated a leaflet saying ‘don’t risk an MP who is not on your side’ featuring a picture of the Prime Minister and Prime Minister Modi of India.
Addressing women and equalities minister Liz Truss Ms Villiers said: “Will she join me in condemning this as divisive and designed to stoke up anti-India and anti-Hindu sentiment?”
Ms Truss welcomed Ms Leadbeater to her seat and went on to “commend her for her dignity in standing up to intimidation during the campaign.”
However she added: “I do agree with (Ms Villiers) about the very divisive nature of the leaflet that she talks about. Politicians should not be stoking division instead we should be working together to unite and level up our country.”
Last week Ms Leadbeater defended the pamphlet as she said Batley and Spen “need an MP who can represent the constituents of this area on local, national and international issues.”
While the campaign was still ongoing she told BBC News: “I don’t think it is a mistake. I think that leaflet is about human rights and it’s about having an MP who is going to speak out on the human rights issues in Kashmir.
“And, sadly, we’ve got a Prime Minister who isn’t doing that, he isn’t challenging the Indian government around these issues. So you need someone who can, and that’s exactly what I would do.”