Jo Swinson has won the Lib Dem leadership contest, beating former coalition cabinet minister Ed Davey.
The result of the contest was announced to party MPs and activists in central-London on Tuesday afternoon.
Speaking after her victory was confirmed, Swinson said to cheers: “I will do whatever it takes to stop Brexit.”
Swinson, 39, becomes the first female leader of the party and will host a rally this evening to celebrate.
She won 63% of the vote in the contest - beating Davey by 47,900 votes to 28,000.
Swinson takes over from Vince Cable as the pro-EU party enjoys a surge in support.
She said she was now a “candidate for prime minister” and fully intended to fight the next election on the belief the party could form the next government.
Swinson’s first electoral test will come in the Brecon and Radnorshire by-election on August 1.
One poll suggested the Lib Dems are on course to snatch the seat from the Conservatives and add to its current crop of 12 MPs, with the party on 43% ahead of the Tories on 28%.
Swinson said today: “Tomorrow, Boris Johnson is likely to take the keys to Number 10 and set us on a path to a damaging no-deal Brexit.
“Stopping Boris, and stopping Brexit is my number one priority as leader.
“That job starts with winning the Brecon & Radnorshire by-election to bring Jane Dodds to Parliament, cut Johnson’s wafer-thin majority and take us a step closer to stopping Brexit.”
The Lib Dems brought forward the announcement of their new leader to today after it was initially scheduled to clash with Johnson’s expected victory in the Tory leadership contest tomorrow.
Ian Lavery, the chairman of the Labour Party, said Swinson “sat at the top table of the coalition government and voted for vicious attacks on the most vulnerable and tax cuts for the super-rich and big businesses”.
“Austerity couldn’t have happened without Liberal Democrats – leading to shocking levels of child poverty, the tripling of tuition fees, a homelessness crisis and rising food bank use,” he added.