The by-election for disgraced former Cabinet minister Chris Huhne's Eastleigh seat will take place on February 28, it has been announced.
The contest - which will pit the coalition parties against each other - was sparked when Huhne stepped down from parliament after pleading guilty to dodging punishment for speeding.
Liberal Democrat Chief Whip Alistair Carmichael is due to move the writ formally triggering the by-election on Thursday, a party spokesman said.
Lib Dem president Tim Farron, who has visited the constituency, said: "Liberal Democrats have a proud 20-year record of action in Eastleigh, fighting for people in both local government and in Westminster.
"The people of Eastleigh should have the opportunity to elect someone to serve the constituency as soon as possible.
"This is going to be a tough fight, and clearly a fight that's between Liberal Democrats and Conservatives. I know that we will be up to the task - and I am confident and determined that we will win."
Deputy Prime Minister Nick Clegg is expected to use a speech to fire the starting gun for the contest, with the party's candidate set to be selected at a private hustings on Saturday.
The Lib Dems have held Eastleigh, in Hampshire, since winning the constituency in a 1994 by-election. Huhne had a majority of 3,684 at the last general election.
The party has not lost a seat in a by-election for more than 50 years.
However, the Conservatives have signalled they will go all-out to achieve the 3.6% swing which would secure a morale-boosting victory. There is speculation that the Tory candidate will be Maria Hutchings, who fought the seat in 2010.
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