The race to replace Sir Vince Cable will come to a close on July 23 when a new Liberal Democrats leader will be announced.
Sir Vince said it was time for a “new generation” at the top of the party but pointed to expected gains at the European elections as a sign that “we are in an excellent position” to lead a centre-ground movement in British politics.
Deputy leader Jo Swinson is firm favourite to succeed 76-year-old Sir Vince, who announced the start of the contest in an email to party members.
He said membership was at a record level and the party had enjoyed the best local election results in its history.
“There are major challenges ahead. One is to win, finally, the battle to stop Brexit.
“Our campaigning has given hope; now we need to secure a referendum in Parliament, and then win it.
“Another is the opportunity created by the conflict and decay within the two main parties to build a powerful, liberal, green, and social democratic force in the centre ground of British politics.
“We are now in an excellent position to lead such a movement.”
The party only has 11 MPs and the leadership rules require any candidate to have the support of 10% of their colleagues in the Commons and 200 members spread across at least 20 local parties.
Nominations close on June 7.
The contest will be taking place as the Tories also choose a new leader following Theresa May’s decision to resign.