The outgoing chair of the Scottish Labour Party said "it has been an immense privilege to lead" the part during the "momentous time in Scottish politics."
In her resignation letter, Johann Lamont said she believed Labour Party is the "only chance for better Scotland."
"Despite the serious challenges we face, I strongly believe that the Labour Party is not only our best chance of achieving a better, fairer Scotland, it's our only chance," she said.
Labour former first minister Lord McConnell told BBC Radio Scotland's Good Morning Scotland programme he was "very, very angry" and said UK party leader Ed Miliband had questions to answer.
Outgoing Scottish First Minister Alex Salmond said:
Outgoing Scottish Labour Party leader Johann Lamont has accused the Westminster leadership of treating the party in Scotland as a "branch office".
Announcing her resignation, Ms Lamont told the Daily Record:
Tony Blair has denied saying Ed Miliband cannot lead Labour to victory at the General Election because he has "failed to connect with voters".
The Daily Telegraph claimed the former prime minister had told friends privately of his concerns.
The paper quoted a source as saying :"The Conservatives will be the next government because Labour has failed to make a good case for itself. That is what Tony thinks."
But Blair's office were quick to dismiss the alleged quotes and tweeted that the story "does not represent my view".
It added: "Ed Miliband and the Labour Party can and will win the next election."