Analysis of opinion polls over the last 18 months by pollsters JL Partners show Sunak has managed to further damage the party’s popularity since replacing Truss in October last year.
She only lasted seven weeks as prime minister after her disastrous mini-budget led to economic chaos.
At that time of her departure from Downing Street, 63% of those who backed the Conservatives in 2019 - around 6.2 million - said they would do so again at the next election.
But by the end of last month - after more than a year of Sunak as PM - that had dropped to just 59%.
Amazingly, the Tories have lost the support of 520,000 of their former voters since October’s Conservative conference.
The main beneficiaries have been the Labour Party, but the right-wing Reform UK is now backed by 15% of 2019 Tories.
In their report, JL Partners said the electoral coalition which put Boris Johnson into No.10 with an 80-seat majority nearly four years ago “has imploded”.
They said: “Between the fall of a disgraced Johnson, the debacle that was the Truss administration’s mini-budget, and the bungled attempts of Sunak to steady the ship, the Conservative vote is in its weakest state yet.
“Critically, Conservative 2019 vote retention is now at an even lower level than after Liz Truss’ mini-budget last autumn.”
The findings are yet another blow for Sunak, who has seen his numerous attempts to turn around the Tories’ fortunes fail.
Despite watering down the Tories’ net zero commitments, announcing a raft of new policies at his party conference and unveiling tax cuts in the Autumn Statement, they are still trailing well behind Labour in the polls.