Supporters of her rival Rishi Sunak insist their candidate will not pull out of the race before its conclusion on September 5.
But Sir John Curtice, professor of politics at Strathclyde University, suggested there was just a 5 per cent chance he could win.
He would be "extraordinarily surprised" if Ms Truss, the foreign secretary, does not win the keys to No 10, he told the Times newspaper.
“Something would have to happen. Truss would have to foul up in some spectacular fashion. Even then it might be too late,” he said.
A recent YouGov poll with Sky News showed Ms Truss still had a 32-point lead over Mr Sunak, similar to that seen in a separate poll of Conservative party members by the ConservativeHome website earlier this week.
Allies of Mr Sunak insist the results do not reflect their experiences on the campaign trail.
But Sir John said: "Unless there is something that’s gone seriously astray this time it is difficult to believe that Sunak is going to make it."
Mr Sunak could gain more support among undecided party members, he suggested, meaning Ms Truss could win by less than 60 per cent of the vote.
"But below 50 it would count as a much bigger error in the polling than we’ve seen previously in the 2015 general election," he said. "There’s a 5 per cent chance that Sunak could win it. Something would have to happen. Truss would have to foul up in some spectacular fashion. Even then it might be too late.
"I would be extraordinarily surprised if she doesn’t win. The evidence in front of one’s eyes all points you in the same direction."
He added that the foreign secretary "has a very clear narrative which clearly resonates with her audience, she sticks to her guns, she doesn’t change her mind. She is still as much in favour of tax cuts as she was six weeks ago, she is Johnsonian in her style. The fascinating question is how well it will all survive once she’s got the job."
The next prime minister will be decided by around 160,000 Tory party members, many of whom are thought to have already voted.