A new opinion poll put the party on just 20%, a staggering 27 points behind Labour.
The last time their numbers were so low was in October 2022, just before Truss was forced from office after just seven weeks as PM.
The latest blow for Rishi Sunak came barely an hour after he saw off a Tory rebellion to win MPs’ backing for his Rwanda plan.
Just 11 Conservatives voted against the Safety of Rwanda Bill, which the prime minister hopes will finally lead to deportation flights to the east African country taking off.
According to the YouGov poll for The Times, support for the Conservatives has dropped by two points from 22% to 20% in the past week, while Labour have gone from 45% to 47%.
Worryingly for Sunak, support for the right-wing Reform UK has also gone up from 10% to 12%.
A separate YouGov earlier this week said Labour were on course to win a 120-seat majority at the general election later this year.
Tory MPs had hoped that replacing Truss with Sunak would turn around their political fortunes.
But despite a raft of pledges, relaunches and policy announcements since entering No.10, the former chancellor has failed to dent Labour huge poll lead.