Jo Swinson has been elected leader of the Liberal Democrats.
The former business minister beat rival Sir Ed Davey in the race to take control of the pro-Remain party.
The MP, who is the first-ever millennial leader of the party, will replace the incumbent Sir Vince Cable, who announced his decision to step down earlier this year.
The Lib Dems have 12 MPs – bolstered by ex-Labour and Change UK MP Chuka Umunna’s decision to join them last month.
And they came second in May’s European Parliament elections, winning 20% of the vote.
Following the announcement of her victory, Ms Swinson told party members: “I am delighted, honoured, absolutely over the moon to stand before you as the leader of the Lib Dems – and as the first woman to lead our party.”
She continued: “There are those out there who think liberalism has had its day… Liberalism is alive and thriving.
“In the face of nationalism, populism, the catastrophe of Brexit, the two old parties have failed.
“Our party has been clear on Brexit from day one. We believe the UK’s best future is as members of the EU… I will do whatever it takes to stop Brexit.”
Ms Swinson has previously suggested, if Boris Johnson becomes prime minister on Wednesday, then her party could be boosted even further.
The East Dunbartonshire MP said she could lead the Lib Dems to victory in a general election.
“We could be better than kingmakers,” she told Sky News’ Sophy Ridge on Sunday.
“I think our politics are volatile at the moment, I think predictions are a bit of a mug’s game in politics.
“I do not put any limit on our ambitions for the Liberal Democrats because our country needs a liberal alternative.”
She also ruled out joining forces with Labour leader Jeremy Corbyn in a future coalition.
“Absolutely not,” Ms Swinson said. “He’s a Brexiteer and is a danger to our country.”
Questioned on whether the party would consider not fielding candidates in certain seats to ensure pro-Remain candidates were elected, Ms Swinson said that the party would be open to the possibility.
“This is about what is the right thing for our country and if you look at what is on offer from Boris Johnson, whether he ends up doing some kind of deal with Nigel Farage, that places our country in serious threat,” she said.
“So at the last election, we didn’t stand in Brighton Pavilion against Caroline Lucas.
“These are decisions that are rightly made locally, but I think that we should be open minded to how we do that to be able so stop Brexit and be able to create the positive, liberal alternative for our country.”
:: Who is Jo Swinson?
The former marketing manager moved into politics in 2005, serving as an MP for 10 years before re-joining the Commons in 2017.
She was a senior aide known as a parliamentary private secretary to former Lib Dem leader and deputy prime minister Nick Clegg the party’s time in government.
During her two years outside politics, she founded her own consultancy business specialising in workplace diversity.
When she returned she became deputy Lib Dem leader.
Ms Swinson made parliamentary history earlier this year by becoming the first person to take a baby into a Commons debate.
She is married to former Lib Dem MP Duncan Hames.