Sir Vince Cable is the new leader of the Liberal Democrats, after no-one stood against him.
The 74-year old Twickenham MP was the only candidate on the ballot paper when nominations closed at 16:00 BST.
The former business secretary said there should be an "exit from Brexit" and he believed the party's call for a second referendum would be vindicated.
He also said he did not intend to be a "caretaker" leader and he would "serve as long as I need to".
Sir Vince, who has been acting and deputy leader in the past, has argued that Brexit is not inevitable.
He assumes the leadership vacated by Tim Farron, who stood down after a disappointing general election in which the party increased its number of MPs from nine to 12 but saw its vote share fall to 7.4%.
Sir Vince paid tribute to his predecessor as he was announced as the new leader, saying Mr Farron had taken over the leadership at a time of crisis for the party and had rebuilt its membership to record levels.
He argued that the Conservatives and Labour had been taken over by "ideologues" and British politics had lost its "basic common sense", moderation and mutual respect - his aim was for the Liberal Democrats to move into that space.
He warned that he feared the UK was "heading for a disastrous outcome" over Brexit, headed by a dysfunctional and disunited government - and said he felt there should be an "exit from Brexit".
The party's main campaign pledge during the election was to give the public the final say on the terms of the UK's exit from the EU in a further referendum, ahead of the scheduled withdrawal date in March 2019.
Although Sir Vince has admitted this message did not "cut through", he has argued that attitudes are beginning to change and that the public mood will come round to the party's position.
He said as the difficulties of Brexit became clearer, Mr Farron's policy would be "absolutely vindicated". He added: "I'm ambitious for this country and I'm ambitious for this party. In difficult times, we have shown enormous resilience but I believe we can fight our way back, break through and make an enormous success of our party and eventually, in government."