Gordon Brown outlines some of his policies for social mobility
Gordon Brown has said improving social mobility in Britain is a "national crusade" and admitted Labour had not yet made enough progress.
In a speech the prime minister said making Britain more "upwardly mobile" was a "great moral endeavour".
He announced a £200 grant for deprived families in England who joined schemes to improve children's development.
Tory leader David Cameron said Mr Brown had failed to focus on "elements of our society that are broken".
In a speech to education figures in Westminster, Mr Brown described himself as "a child of the first great wave of post-war social mobility" but said that these advances stalled in the 1970s and 1980s, with a "lost generation" of "Thatcher's children" left behind.
He said Labour had narrowed the gap in schools between social classes.
But he added: "Although we have already lifted hundreds of thousands of children out of poverty with new tax credits, more people in work, and better public services, the latest figures show we have not made enough progress."
He added: "We will not deny or explain away the figures. We will take them as a spur to action, a call to conscience."
We must set a national priority to aggressively and relentlessly develop the potential of the British people
Mr Brown said there would be a White Paper on social mobility by the end of the year, and announced pilot projects to tackle child poverty - for example by a one-off £200 child development grant for parents to attend and take up services in new children's centres.
It will be trialled in ten areas in England based on a scheme from New York, where parents are rewarded for activities like making sure their children attend health check-ups and get jabs.
Mr Brown also said he wanted to double the number of Teach First placements - which put "the most able" graduates in the toughest city schools for two years.
Two-year-olds in the most disadvantaged areas would be offered free nursery places, he also said.
Some £5m is to be made available to provide financial incentives for both parents to work, because research suggests that children are 60% less likely to be in poverty when both parents have jobs.
Injustice must be tackled, prejudice and discrimination removed and children's aspirations raised, the prime minister added.
"Raising social mobility in our country is a national crusade in which everyone can join and play their part," he said.
He said Britain had to be "far more upwardly mobile", adding: "At its core, this is a great moral endeavour."
The Sutton Trust, which funds projects that provide educational opportunities for children from underprivileged backgrounds, has previously said the government's education policy fails to give poorer children the chance to improve quality of life.
A study conducted for the trust last year concluded that the decline in social mobility seen during the 1970s and 1980s has now flattened off, but shows no sign of reversing.
Conservative leader David Cameron said Mr Brown was wrong to blame Margaret Thatcher for inequalities.
"Children who are leaving school now have been at school under a Labour government for the entire period, at primary and secondary school," he said.
He argued that social mobility was "blocked" because the government had not opened up secondary education, had not adopted tough welfare reforms and not properly addressed drug abuse and homelessness.
"They have not properly focused on elements of our society that are broken," he said.
Child Poverty Action Group chief executive Kate Green said Britain was "in the grip" of a "damaging culture of inequality".
She said: "It is gross inequality that is the enemy of opportunity and social mobility. It is Britain's exceptional gap between the richest and poorest that has created a gulf that can no longer be navigated."
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