The Eastleigh by-election will take place on February 28th, kicking off what is set to be a bitter three-week campaign in Hampshire.
Chris Huhne's resignation as an MP means the Liberal Democrats are defending a seat for the first time since the formation of the coalition government in 2010.
They face a tough challenge from their partners in government, the Conservatives, who are looking to overturn a majority of 3,864 in a seat many expected would turn blue at the last general election.
But the Lib Dems are dug in well in the area, holding all 36 council wards in the parliamentary constituency and a strong reputation as incumbents.
"Liberal Democrats have a proud 20-year record of action in Eastleigh, fighting for people in both local government and in Westminster," party president Tim Farron said.
"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."
The Lib Dems have not even selected a candidate for the seat yet but will do so on Saturday, just 19 days before the by-election takes place, following a private hustings event.
Local party chiefs are warning the Conservatives launched their campaign "weeks ago" and predicted a "bruising battle" ahead.
Commentators were quick to suggest the by-election was being rushed through to minimise the damage the campaign would have on coalition unity in Westminster.
Conservative and Lib Dem MPs have suffered a difficult start to 2013. The parties' backbenchers have been at odds on a range of issues, including boundary changes, the EU in-or-out referendum and, this week, gay marriage.
Lib Dem chief whip Alistair Carmichael will formally move the writ to trigger the by-election in parliament tomorrow, when party leader Nick Clegg is expected to make a bruising speech attacking his coalition colleagues.