Analysis for Sky News by Prof Will Jennings, an elections expert, showed Labour has lost votes across the country in areas with high Muslim populations.
In neighbourhoods with a Muslim population smaller than two per cent, its average vote share has gone up by an average of 1.5 per cent compared with last year.
In council wards where more than five per cent of the population identifies as Muslim, the party has suffered reverses, seeing its support decline by 2.2 per cent.
Momentum, the Left-wing group that supported Jeremy Corbyn, said it showed Muslim voters were punishing Labour for its approach to the conflict in Gaza.
A Momentum spokesman said: “These losses should set alarm bells ringing in Labour HQ. Any party which takes its core vote for granted risks disaster sooner or later.
“Starmer should respond by getting off the fence and calling for a suspension of arms sales to Israel, as other parties have done, and unreservedly condemning Israel’s ongoing war crimes in Gaza and the West Bank.”
It comes after Mr Parker’s campaign was rocked by the eve-of-election revelation that police were looking at claims he wrongly put down where he was living to get on the ballot paper.
West Midlands Police said it was “assessing” allegations that the Labour candidate may have breached electoral law after receiving a complaint from a local Conservative MP.
Official nomination papers show that Mr Parker declared a rented flat in central Birmingham as his address for the election. But he has publicly admitted that his family home is situated in a village a quarter of a mile outside the West Midlands Combined Authority area.
A spokesman for Mr Parker dismissed the claims as “shameful political game-playing”.