Some 15% of Londoners do not have suitable ID to vote in elections, according to research by London Labour.
Young people and those from minority ethnic backgrounds are also five times more likely to be turned away from polling stations, according to a new Electoral Commission report.
Mayor of London Sadiq Khan, who has repeatedly criticised the rollout of the new identification rules, branded it a “cynical assault” on voters’ rights in response to the findings.
Polling by Opinium commissioned by London Labour showed 20% of people aged between 18 and 34 do not have the required ID, compared with 12% in the 35-49 bracket and 13% of 50 to 64-year-olds.
But this rose to 5% among 18 to 24-year-old non-voters and those from minority ethnic backgrounds.
Under a new policy introduced by the Conservative government, this year’s local elections in England were the first time people were required to show identification before collecting their ballot paper at polling stations.
The rules were widened to cover UK general elections from the autumn, and will apply to the London mayoral vote expected next year, in which Mr Khan will face a challenge from Tory candidate Susan Hall.
Passports, driving licences, older or disabled person’s bus passes and 60+ Oyster cards are among the documents permitted.
Critics have argued that the policy runs the risk of deterring younger generations and people from ethnic minorities from voting, but ministers say the changes will help prevent electoral fraud.
Mr Khan is calling for an expansion of accessibility and a better-resourced public awareness campaign to reach voters most affected by the changes.
He said: “People across London and the UK face crucially important elections next year. As things stand there’s a real possibility that thousands of voters will be turned away from polling stations through no fault of their own, which could affect the outcome.
“The evidence is clear that it will be young people, ethnic minorities and those from poorer communities who will be affected most by this cynical assault on voting rights.
“We simply can’t have a situation where thousands of people are locked out of the political process.”
A Department for Levelling Up, Housing and Communities spokesperson said: “These claims are untrue. The vast majority of voters in the polling station – 99.75% – cast their vote successfully at the local elections in England in May.”