The timing of the vote set by Central Bedfordshire Council means that the result will not have an impact on the Conservative party conference, which is being held at the start of that month.
Former culture secretary Ms Dorries was elected five times in the historically safe seat since 2005, each time with a very comfortable majority.
But the Conservatives are wary of any electoral test at the moment as the party trails far behind Labour in national polling.
The by-election is taking place after Ms Dorries resigned as the constituency MP in August.
Despite saying in June she would quit, Ms Dorries subsequently said she wanted to find out why she was refused a seat in the House of Lords.
It was widely thought she would be made a peer by former Prime Minister Boris Johnson in his resignation honours list.