Ed Miliband is launching Labour's biggest voter registration drive in a generation in an attempt to rebuild the support which delivered the landslide election victory of 1997.
Following last week's local elections - which saw the lowest turnout in a decade - he will say the party needs to regain the trust of an electorate which has lost confidence in politicians.
With an estimated six million people already missing from the electoral register, he will say there is "a very long way to go to generate trust, enthusiasm and deep allegiance".
Addressing the Progress annual conference in London, he will say Labour must show it is prepared to stand up for the interests of the many in the face of a Tory-led Government representing only the powerful and privileged.
"We now have an opportunity and we must seize this moment," he will say.
"I want the British people to understand how the Labour Party is changing. To know the true character of the party I want us to be.
"Our work to make that happen is well under way but now must intensify."
Mr Miliband will also mount a fresh attack on the Government in the wake of the latest revelations at the Leveson inquiry, accusing it of having "bent over backwards" to help Rupert Murdoch's media empire while Britain was sliding into a double-dip recession.
"The reason the Government has lost its way so badly in the last few weeks is because they've been revealed for who they are - a government that stands up for the rich and powerful," he will say.
He will insist that Labour in office had never behaved like Culture Secretary Jeremy Hunt, who is accused of secretly assisting the Murdochs' bid for BSkyB.