Boris Johnson is at risk of losing his seat in the next general election amid a surge of younger voters.
Research by centre-Right think-tank Onward found that constituencies with ratios above 1:1 voters aged 20-39 for 60 pluses were likely to be won by parties other than the Tories, potentially costing them a fourth term at the next election.
Some Conservative seats have ratios far higher than the tipping point, suggesting they are vulnerable, the report says.
Those include the London seats of Cities of London and Westminster, Hendon, Chelsea and Fulham, and Johnson’s seat of Uxbridge and South Ruislip.
The research has been revealed after a separate study found just 16 per cent of under-35s said they would currently vote for the Conservatives at the next election.
The age at which people become more likely to vote Conservative than Labour is now 51, up from 47 at the 2017 election and 34 before that.
Recent polling puts Conservative support at the lowest it has been in five years as turbulent Brexit talks and party divisions grip the party.
The party would take 29 per cent of the vote as things stand, giving Labour a 7 point lead on 36 per cent of the vote.