The date for the Rochester and Strood by-election - triggered by Mark Reckless's defection to Ukip - has been set for November 20.
Chief Whip Michael Gove is expected to formally move the Commons writ for the contest later today.
It will be a crucial test for the Conservatives, after fellow defector Douglas Carswell scored a resounding victory in Clacton last week.
The news comes after the Conservatives' shortlist for the by-election was revealed, with two Kent councillors battling to be the party’s candidate.
Kelly Tolhurst, a Medway councillor who represents the Rochester West ward and is cabinet member for education, will vie with Sevenoaks councillor Anna Firth in the selection battle.
There had been speculation that former Olympic rower James Crackell could be in contention but the party has opted instead for local people.
Kelly Tolhurst is a life-long resident of Medway and was elected to the council in 2011.
She is the cabinet member for school improvement. She now runs her own marine survey business in Rochester and has served as a local councillor for over three years.
In a statement, the Conservative association said she had “seen the strain that excessive immigration has put on housing and our local services.”
Anna Firth lives in Sevenoaks with her husband and three children and was brought up in Essex. Formerly a barrister, she now serves on the Sevenoaks council. She was shortlisted for the Thanet South constituency but lost out to Craig Mackinlay.
She is a governor of Tonbridge Grammar School for Girls.
The Conservative association said both were strong candidates to take on Mark Reckless but the choice of who should be the candidaite would be decided by local voters.
Party officials have agreed to a “primary” selection process, under which any voter who lives in the constituency will be able to vote for the candidate, regardless of whether they belong to the party.
The result of the postal ballot will be announced on October 23.
The date of the election announcement is likely to trigger a marked increase in activity by the rival parties, with Conservative leader David Cameron slotted for a number of visits to the constituency.
At the 2010 general election, Mr Reckless won the Kent seat as a Conservative with a majority of 9,953 over Labour. Ukip did not field a candidate.
Voters this time are being invited to quiz the Conservative candidates shortlisted to contest the forthcoming Rochester and Strood by-election at an open hustings meeting.
The public meeting on Wednesday will be held at Rochester’s Corn Exchange and will get underway at 7.30pm.
Admission is free.
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