The former Man United player will campaign with the Labour leader in the weeks and months ahead. He first shared a platform with Sir Keir last week at a fundraising event in Manchester.
There are claims the former footballer turned pundit could even be hoping to enter politics himself.
Mr Neville announced he had joined the Labour Party in January.
He has been vocal in expressing his political opinions on social media and lambasting Prime Minister Boris Johnson.
In December, he labelled him “the worst kind of leader”.
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Sir Keir, who is an Arsenal fan, is thought to believe the ex-England player could help boost his electoral chances this May and at a future general election.
“They were very relaxed around each other on stage – they seem to have a really nice relationship,” a Labour source told City AM.
“We’re really pleased he’s a member and that he wants to use his membership.
“But there have not been any talks with him about standing as an MP.”
Labour faced a mauling at the local elections last year.
While the party increased its majority in strongholds such as London and Manchester, it saw a significant decline in votes elsewhere.
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The former right back turned left-winger has previously suggested he is unlikely to end up in Westminster for fear he’d get “eaten alive”.
He told the BBC earlier this year: “I say to myself sometimes I’m able to communicate, I know what I want, I have got a business mind, I can manage people in respect of businesses.
“But I just think I’d get eaten alive.
“I don’t know what Whitehall is like, Westminster is like.
“I can only imagine what you have to do there to survive and I don’t play the game, so how can I get in?
“So when I ask myself the question do you want to dip your toes in, I can’t dip my toes in because I’m all or nothing.
“I go in and I’m going in.”
He added: “I do believe we need a progressive Labour Party, but one that not just looks after what would be the left side of the party, it has to come towards the centre.”