Success of pro-Corbyn candidates marks clear shift to the left as party prepares in key marginals for possible snap election

New evidence of Labour’s shift to the left has emerged as candidates backed by Unite and Momentum are set to dominate the battle for marginal seats in the next election.

More than a third of the candidates selected to fight marginals for Labour were backed by, or closely aligned with, the leftwing pressure group Momentum, while nearly half were backed by Unite, the union closest to Corbyn.

Labour has chosen its candidates for 96 marginals – held by opposition-party MPs with majorities smaller than 10,000 – in preparation for a snap election. Should it win all of them, it would have a majority of 28 in parliament.

The figures, analysed by the Observer, show that the next raft of Labour MPs is set to be considerably more supportive of Corbyn and his wing of the party, as candidates further to the right are struggling to win selection contests. In total, Momentum endorsed 67 candidates in the 96 selections, winning with 37 of them, while Unite endorsed 73 candidates and won with 47.

In many cases, Momentum, Unite and other trade unions have rallied behind the same candidate – such as Greg Marshall in Broxtowe, the seat held by Anna Soubry with a majority of just 863, and Hannah O’Neill in Milton Keynes South.

But in other areas they have thrown their weight behind opposing figures. In Truro and Falmouth, Unite-backed Jen Forbes beat Momentum’s pick, Anna Gillett, for the selection.

Momentum-backed candidates have won selections in several seats where Unite did not put forward a nomination. They include Tina McKay in Colchester, Laura McAlpine in Harlow and Jackie Schneider in Wimbledon.

Unite runs a well-coordinated campaign to get its preferred candidates selected in target seats; nearly two-thirds of sitting Labour MPs are linked to the union. Momentum’s operations, on the other hand, are patchy, with its head office and local branches sometimes endorsing different people. In Scarborough and Whitby, Steve Howley – who had the support of Unite and Momentum’s head office – was defeated by Hugo Fearnley, who was backed by the local Momentum branch.

Corbyn critics not backed by Momentum or Unite have won a handful of selection battles, including Peter Lamb in Crawley and Sally Keeble in Northampton North.

Analysis of the selections in 96 marginals reveals regional differences in Momentum’s reach. Half the selected candidates in the east Midlands, the east of England, London, and Yorkshire and the Humber are closely linked to Momentum. The proportion falls to one-third in the south-west, south-east, north-west and West Midlands.

In Wales, just five selections for 12 marginal seats have been made, three of them backed by Unite and one by Momentum. In Scotland, where 18 selections have been made, nine were backed by Unite. Momentum and its affiliate the Campaign for Socialism are linked to several winning candidates in Scotland, including Matt Kerr in Glasgow South West, Rhea Wolfson in Livingston, and Cara Hilton in Dunfermline and West Fife.

Nationally, Momentum’s campaigning is focused on the constituencies of leading Tories with narrow majorities. The most vulnerable targets are Justine Greening, Amber Rudd and Soubry, while others such as Iain Duncan Smith, Nicky Morgan and Boris Johnson will require larger swings.

Two-thirds of the candidates selected so far to contest marginals for Labour are women.

A spokesman said: “We’re delighted that Momentum-backed candidates have won in more than half of the selections we’ve contested. We’re proud to back a new generation of up-and-coming Labour MPs, including 24-year-old Ali Milani, campaigning to unseat Boris Johnson, and Faiza Shaheen, the working-class east Londoner running against Iain Duncan Smith in Chingford and Woodford Green.”