The most important by-election of 2014 will be held on 20 November, the assistant political editor of The Times revealed on Tuesday.
Philip Webster said on Twitter that the fight for the Rochester and Strood seat is being played by the Tories "for as long as they can."
The by-election will be a close fight between the Conservatives and Ukip, after it was triggered by the defection of Mark Reckless MP to Nigel Farage's party.
Reckless will campaign for re-election to the seat for Ukip, hoping for the same success seen by Douglas Carswell, who became Ukip's first elected MP last week, and who is another Tory defector.
Farage the result of Thursday's poll in Clacton, which Carswell won for Ukip by 12,404 votes, was "absolutely phenomenal".
Tory and Labour MPs have called for a response to Ukip's surge in popularity, and Prime Minister David Cameron is said to be under "intense pressure to crush Ukip" in the by-election, after senior party members blamed him for the Eurosceptic party's victory in Clacton.
After losing the previously safe Tory seat of Clacton, the battle for the seat of Rochester and Strood, Kent, will be a critical test of the prime minister's authority ahead of the 2015 general election.
Sources inside the Conservative party have said they are planning a campaign in Rochester to recruit Labour and Lib Dem votes, who dislike Nigel Farage, to vote Tory in a 'keep Ukip out' campaign, reports The Daily Mail.
Cameron has promised to "throw everything he can" at the contest, which is expected to be won by a much smaller margin than Clacton.