A Labour party plan that would save Jeremy Corbyn’s seat, which is threatened by boundary changes, has resulted in a furious reaction from one of the leader’s closest allies whose own constituency is to be split into three under the proposals.

Kate Osamor, the shadow cabinet minister for international aid, surprised colleagues by turning up at a public meeting at Havering town hall to oppose her own party’s submission on the changes, the Guardian can reveal.

The MP said she was unhappy that Labour was suggesting her own constituency of Edmonton, which was barely changed in the original proposals from the Boundary Commission, should be broken up.

A leaked document outlining Labour’s response to the boundary changes shows that the central aim is to protect Corbyn’s Islington North seat after the initial proposals divided it up.

The boundary commission has one chunk of Islington North going into a new constituency of Finsbury Park and Stoke Newington, while another slice moves into Diane Abbott’s Hackney seat.

The party’s counter-proposal keeps the constituency intact but instead hits other MPs, including Osamor and David Lammy, who represents Tottenham.

The submission admits there is an issue, saying: “We recognise that these proposals divide communities in Edmonton and Tottenham, as well as Crouch End from Hornsey and Highgate. On balance, however, we believe that the consequence of the commission’s proposal to include part of Hackney in a seat with part of Islington is that much more disruptive proposals are made in the rest of north London which break ties more seriously.”

One person present at the hearing told the Guardian that Labour politicians there were surprised to see Osamor turn up to tell a panel that she was unhappy about the plans and would instead back Lammy, who has argued that his community is hundreds of years old and should not be split up.

He has written to the commission to say its initial proposals “maintain the integrity of the current constituency boundaries”, and should be maintained over Labour’s proposals.

The MP wrote a long letter in which he claimed that dividing Tottenham would be risky in terms of cohesion and gang activity, and even included poetry that was 700 years old to underline the “separate, independent and distinct nature of Tottenham”.

A source in the Labour party said: “It’s so obvious the the party is putting leaving Jeremy’s seat unchanged above every single other consideration. Everyone knows he can just take his pick of whatever Islington seats the boundary commission come up with. Splitting Tottenham is a total nonsense, and Kate knows that as well as anyone.”

It is understood that Osamor was sympathetic to the idea of protecting Corbyn but was furious about the way that the party put it into action - complaining to the individual in charge of the submission for the party. She hoped that a number of colleagues could instead all take a smaller hit in order to make the plans work.

The person present at the hearing said Osamor appeared particularly upset about her Labour colleague, Joan Ryan, who represents Enfield North.

Osamor declined to comment when approached by the Guardian.

A Labour party spokesperson said: “As part of the Boundary Commission’s consultation process, the Labour party have put forward counter-proposals that better respect local community links and ties. These proposals have been agreed by the NEC after a wide-ranging consultation with local parties.”