The former culture secretary also suggested she might not even vote Tory.
Dorries announced on Saturday she was finally actually resigning as an MP, more than two months after she first said she would do so.
It means a by-election will soon take place in her Mid Bedfordshire seat, with both Labour and the Lib Dems hopeful of overturning her 24,664 majority.
In an interview with TalkTV on Sunday afternoon, Dorries was asked if the Conservatives could win the next general election with Sunak as leader.
“I have to say my answer to that question is no, I don’t think so,” she said. “It’s very unlikely.”
Dorries - a close ally of Boris Johnson - said: “The Conservative Party is broken.”
And asked if she would vote Tory, Dorries suggested she might not. “You are asking me a question I don’t want to answer,” she said.
Hear Nadine Dorries' response when asked if the Conservative Party can win the next election with Rishi Sunak, and would she vote for him.@nickdebois |@NadineDorries | #NadineDorries#TalkTVpic.twitter.com/lcWjEkbwZV
— TalkTV (@TalkTV) August 27, 2023
Dorries dramatically announced in June she was quitting parliament “immediately” after she was denied a seat in the House of Lords.
But she delayed her formal resignation, preventing a by-election from taking place before the summer.
It means Sunak faces the prospect of another embarrassing defeat at the polls.
Dorries had come under increasing pressure to stand down from Tory MPs, frustrated at the fact she had remained in place despite not having spoken in the Commons chamber since June 7, 2022.
Sunak had said Dorries’ constituents were not being “properly represented”, while other Conservatives lined up to tell her to go.
In her resignation statement, Dorries attacked Sunak’s leadership of the party. “What exactly has been done or have you achieved? You hold the office of prime minister unelected, without a single vote, not even from your own MPs,” she said.
“You have no mandate from the people, and the government is adrift. You have squandered the goodwill of the nation, for what?”
Johnny Mercer, the veterans minister, on Sunday defended the prime minister from the “personal attack”.
“I don’t think we are on track to lose that by-election. I think we’ve got a good candidate up there,” he told Times Radio.