Mr Cameron says he wants to help the have-nots as well as the haves

He wants to reform social security to encourage "aspiration", claiming the "clunking tax credit system" just punishes poor families.

He would lift 300,000 children out of poverty and end the revolving door of people "flitting in and out of work".

But Work and Pensions Secretary Peter Hain dismissed the plans as uncosted.

Mr Cameron's speech comes just 10 days after Mr Brown announced there would be no election, and at a time when the Conservatives are enjoying a resurgence in popularity.

He told an audience in north London that while the Tories want to help middle class aspiration, "we cannot forget the have-nots".

Over-taxed middle classes

"Aspiration is not about class, background or position," he said.

"Everybody dreams of rising up in the world and everybody dreams of giving their children a better life.

"I don't care where you started out in life; my mission is to help you rise higher.

"But I do care about this: the fact that for millions of our fellow citizens today, rising higher is not a question of a better job, a better home or a better holiday.

"It is a question of ever having a job, ever owning a home or ever having a holiday."

I do not doubt for one moment Mr Brown's sincere desire to remove the scourge of poverty from our land, but he must see that his methods have failed and it is time for change David Cameron

He added: "So yes, we must help the haves to have more, yes we must back the aspirations of our over-taxed, over-burdened middle classes, but a modern aspiration agenda means helping the have-nots to have something.

"If we do not succeed in that mission then I tell you frankly, that we will all be poorer."

Mr Cameron said the government had undermined families by punishing couples who stayed together.

The "clunking" tax credit system had caused misery among the poorest and wasted billions in overpayments, error and fraud.

"I do not doubt for one moment Mr Brown's sincere desire to remove the scourge of poverty from our land," he said.

"But he must see, as we can all now see, that his methods have failed and it is time for change."

Voluntary sector

Mr Cameron said there were 600,000 more people in "deep poverty" than in 1997.

Tories would take a more "holistic and sophisticated" approach" to "make British poverty history".

They would lift 300,000 children out of poverty by increasing the working families tax credit for couples so that it did not benefit them to live separately.

This would be paid for by stopping the "revolving door of people flitting in and out of work," Mr Cameron said.

And there would be less bureaucracy so the voluntary sector could take a wider role, he said.

Spending 'black hole'

He would also boost the "professionalism and status" of social workers.

Using his own personal experience of having a disabled son, Ivan, he said his family's social worker was "a star".

"I know how hard they fight to get you the funding and the back up you need," he said.

But Mr Hain mocked the Conservatives for coming up with uncosted "quick fixes".

"The notion the Tories could move more than a million people from benefits into work, in order to fund their tax cuts, without any detailed policy proposals for doing so or any up front investment will be seen for what it is - a massive black hole in their spending plans," he said.

TUC general secretary Brendan Barber said: "David Cameron may have strong rhetoric on reducing poverty, but his policies will have the opposite effect.