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The Labour leadership became a four-horse race tonight with Yvette Cooper declaring she will run for the top job.

The Shadow Home Secretary joins Andy Burnham, Chuka Umunna and Liz Kendall in the battle to replace Ed Miliband

In an exclusive article in tomorrow's Daily Mirror, the mum-of-three pledges to “make life better for families”.

She says Labour lost the election because it failed to convince voters it “had the answers to match up with their ambitions”.

The Shadow Home Secretary, whose husband Ed Balls lost his seat last week, also warns against the party trying to turn back the clock.

Mr Burnham, the early favourite, used a video message today to announce he was throwing his hat in the ring.

In a swipe at the left and the right of the party he said Labour could not just speak to one group of voters.

The Shadow Health Secretary said: “Our challenge is to rediscover the beating heart of Labour.”

Labour’s ruling National Executive Committee said the result of the leadership and deputy ­leadership will be announced on September 12.

Candidates need the nominations of 15% of the parliamentary party – 35 MPs – by June 15 in order to stand.

Contenders expected to run for the deputy leadership include ­Caroline Flint, Tom Watson, Stella Flint and Angela Eagle.

Meanwhile, Unite union chief Len McCluskey hit back at Lord Mandelson calling for the return of Blairism. He said the answer was not to “party like 1997”.

He added: “Nor do I accept Labour lost because it was too left wing.”

Yvette Cooper says:

'I don’t want to be the next leader of the Labour Party just because there’s a vacancy, I want to make life better for Britain’s families.

Millions of Mirror readers woke up to a bitter disappointment last Friday morning – the prospect of five more years of Tory Government.

But let’s face facts. Labour lost because we didn’t convince enough people in all parts of the country that we had the answers to match up with their ambitions.

Our promise of hope wasn’t strong enough to drown out the Tory and UKIP voices of fear.

That’s what we need to change. And that’s why today I’m putting forward my name to stand as next leader of the Labour party.

I’m proud of the things Labour has done together – building the NHS, bringing in the National Minimum Wage, expanding universities, creating Sure Start. And helping young people get jobs even in the face of a financial storm.

But as a mum of three children I’m in politics not because I'm proud of the past, but because I care so much more about the future.

All of us as parents want the best for our kids – and we worry about what jobs they’ll get to do, whether they’ll be able to afford a home and family of their own.

We want them to be ambitious, to have great opportunities – and above all for all our family to be happy and safe.

Yet too few people feel confident about their own and their children’s future in such a fast changing world.

The digital age is creating amazing opportunities, but many jobs are out of date.

Long gone are the jobs for life. But gone too is the security of a life-long trade or skill.

Under the Tories, for too many people pay has fallen, living standards have been squeezed and they have been left behind.

So yes, people want to know where the good quality jobs of the future are coming from - whether they have a chance at them, and whether they will be in their own town or just miles away in the city.

In the end, Labour didn’t convince enough people that we had the answers.

They liked a lot of what we had to say, about raising the minimum wage, expanding childcare, cutting tax for low paid workers and banning bad zero hours contracts. But for many people it wasn’t enough to give them hope and confidence we could match all their ambitions for the future.

And when there’s too little hope, optimism or confidence, the politics of anger, fear and division takes over – that’s what the Tories, the SNP and UKIP all exploited and campaigned on in this election.

The fracturing of politics reflects the fracturing of our country and our communities. Divided between rich and poor, north and south, city and small town. And it leaves Britain a darker, narrower place.

But that’s why Labour needs to be bigger in our appeal, bolder in our ambitions and brighter about the future.

Going back to the remedies of the past, of Gordon Brown or Tony Blair, won’t keep up with the way the world has changed.

We need a Labour party that moves beyond the old labels of left and right, and focuses four-square on the future. Credible, compassionate, creative, and connected to the day to day realities of life.

The Tories don’t have any of the answers for Britain’s future. All they will do is divide us, holding families back, widening inequality, helping only the richest in the country while everyone else gets left behind.

So that is why the next Labour prime minister needs clear purpose; a clear view of the problems we face and a clear priorities for the future.

I don’t want to be the next leader of the Labour Party just because there’s a vacancy, I want to make life better for Britain’s families.

It isn’t enough to say we can stop bad things happening, we need to show how good we can be for people too.

This four-month leadership election can’t be just a debate about the future of our party as some have suggested. It has to be a debate about the future of our country.‎

I’m in politics to make change happen. Because I believe we are an incredible country, capable of achieving anything when we put our minds to it and pull together.

And when we’re determined, together we’re unstoppable.'