Jeremy Corbyn’s local Labour party has nominated Keir Starmer for the Labour leadership in a surprise result that confirms he is picking up crucial support from the party’s left.

In a blow to rival Rebecca Long-Bailey, considered the most leftwing candidate and closest to Corbyn’s leadership, the leader’s constituency Labour party narrowly voted to support Starmer’s campaign.

Local members in Islington North said Starmer won the nomination by three votes more than Long-Bailey. Some of Starmer’s supporters were said to be pushing his connection to the radical leftwing publication Socialist Alternatives, which he edited in his early 20s.

Support from Corbyn’s north London stronghold cements Starmer as the clear frontrunner before a crucial week of the campaign. He has the support of more than twice the number of constituency Labour parties than Long-Bailey, his nearest rival.

Tensions bubbled to the surface on Saturday at a hustings in Nottingham, when Emily Thornberry appeared to take a swipe at Long-Bailey’s disputed claims to have been up all night and fuelled by pizza as a result of the mass frontbench walkout designed to topple Corbyn in 2016. To gasps from the audience, Thornberry said: “I know some people had to be up late eating pizza but I did all these difficult jobs at the same time in support of the leader in the way he needed to be supported.”

Long-Bailey responded to the jibe, saying: “Oh, miaow. We love each other really. I was working very, very hard, I’ll have everybody know. We did have to eat pizza.”

Responding to the question that prompted the clash, which was whether it was acceptable for a Labour MP to attack a Labour council, she said: “In terms of attacking colleagues, we shouldn’t attack each other publicly under any circumstances.”

Starmer pulled out of the hustings because his mother-in-law remains in a critical condition in hospital after an accident last month.

The new Labour leader will be announced on 4 April.