Sir Keir Starmer will use his first major speech of the election campaign to tell voters: "I will fight for you."
The Labour leader will say he recognises many people have not yet decided how they will vote on 4 July despite being "fed up with the failure, chaos and division of the Tories".
He will admit many "still have questions about us" and will be asking whether Labour has "changed enough" and if they can be trusted with the economy, immigration and security.
"My answer is yes you can - because I have changed this party. Permanently," he will say.
"This has been my driving mission since day one. I was determined to change Labour so that it could serve the British people."
Sir Keir will accuse Rishi Sunak of governing to appease sections of the Tory party, not for the whole country.
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He will put security at the heart of his speech as the prime minister seeks to get one up on Labour after announcing a new national service programme for 18-year-olds.
Sir Keir will say Labour has changed under him and is ready to meet the "core tests" the British people set for government - economic security, border security and national security.
"Make no mistake, if the British people give us the opportunity to serve, then this is their core test. It is always their core test," he is set to say.
"I haven't worked for four years on this, just to stop now. This is the foundation, the bedrock that our manifesto and our first steps will be built upon."
Ahead of his speech, in southeast England, Sir Keir released a video featuring his childhood home and pictures of his parents, revealing his family were hard up at times and had their telephone cut off.
Appealing to people struggling through the cost of living crisis, he says he knows what they are going through and knows what it is like to struggle to make ends meet.
"I grew up working class, I've been fighting all my life and I won't stop now," he says in the video.
During his speech today, he will also emphasise how he has fought to change the Labour Party since becoming leader.
He will say: "There are always people who say, don't do that, don't go so fast.
"But whenever I face a fork in the road, it always comes back to this: the golden thread: country first, party second."
He will also say the policies his party are putting forward are "a new path for our country, a plan that will turn the page, deliver stability and change".
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Tap hereEchoing his shadow chancellor, Rachel Reeves, he will also say all their policies are "deliverable, fully-funded and ready to go".
"A clear direction, not the endless spinning around that successive Conservative governments have subjected our country to," he will conclude.
Reacting to the speech, Richard Holden, the Conservative Party chairman, said Sir Keir "doesn't say anything" in it.
"Sir Keir Starmer has been Labour leader for four years but has failed to set out a clear plan to secure Britain's future," he said.
"All he has managed to achieve is to break the 10 promises he made when he ran for the leadership and create a £38.5bn black hole in his spending promises, meaning Labour would have to put up taxes by £2,094 on every hardworking family.
"The choice is clear: stick with the plan that is working and take bold action for a safer, more secure future with Rishi Sunak. Or, go back to square one with Sir Keir Starmer and the same old Labour Party."
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