Sir Menzies Campbell dramatically quit as leader of the Liberal Democrats, blaming Gordon Brown's decision to cancel a snap election for his decision to step down.
His abrupt departure "with immediate effect" leaves his deputy Vince Cable in charge of the party.
Arrangements to elect a successor will be announced on Tuesday, party President Simon Hughes said.
Sir Menzies said in a statement: "It has become clear that following the Prime Minister's decision not to hold an election, questions about leadership are getting in the way of further progress by the party.
"Accordingly I now submit my resignation as leader with immediate effect. I do not intend to hold a press conference or to make any further comment."
The first announcement of Sir Menzies' departure came from Mr Hughes and Mr Cable, after a day of speculation and drama at Westminster following the party's plunge to only 11% in the polls.
Mr Hughes read out a statement on the steps of the party's HQ, saying: "Ming Campbell has submitted his resignation as the leader of the Liberal Democrats with immediate effect.
"For the last two years Ming has given huge purpose and stability to our party. He has led the professionalisation of the party and he has led the very successful preparations for the general election whenever it comes."
Favourites to succeed Sir Menzies will include home affairs spokesman Nick Clegg and environment spokesman Chris Huhne.
Prime Minister Gordon Brown described Sir Menzies as "a man of great stature and integrity who has served his party and country with distinction", while Tory leader David Cameron added his tribute, saying: "Sir Menzies Campbell has had a distinguished parliamentary career and is a fine public servant."
