Analysis By Nick Assinder Political correspondent, BBC News website

Labour politicians once had what they believed was the ultimate weapon when attacking Tories over public services.

State schools expected to provide held for special needs

If they were so committed to state provision, most crucially education, why did they so often chose private schools for their own children?

The implication was simple - that the Conservatives could never be trusted to improve state education because they regularly bought their children out of it.

Then Tony Blair came to power in 1997 claiming "education, education, education" was his priority and promising to boost spending on, and standards in, the state sector.

But a number of Labour ministers have faced charges of hypocrisy for opting their children out of the system or, as the prime minister did, seeking selective schools.

Now it has emerged that former education secretary, Ruth Kelly, has taken one of her children out of state education because the child has substantial learning difficulties which, she says, can best be dealt with in the private sector.

Two-tier system

Ms Kelly was education secretary between December 2004 and May 2006. During that time some special schools closed in a process which had also taken place under the previous Conservative administration.

The idea was to see children with learning difficulties taught, as much as possible, in mainstream state schools.

Ms Kelly has provoked backbench anger

For some on the Labour benches that is already enough to leave Ms Kelly open to charges of buying privilege and rejecting a core Labour principle.

Her critics claim it only adds to the impression that the state sector is failing such pupils and that a two-tier system - one for the well-off and another for the rest - not only exists but is being encouraged by a senior government minister.

The fact that the �15,000-a-year school Ms Kelly has chosen claims to help prepare children with learning difficulties pass exams for top public schools will not have helped boost her case on the Labour backbenches.

Social privilege

Her supporters, who perhaps embarrassingly include Tory leader David Cameron, have defended her decision, saying any parent would want to do what was best for their children.

The prime minister's official spokesman has stressed that long-standing government policy has been to give parents choice and to allow a mix of private and state provision.

And Ms Kelly has insisted her other children will continue in the state sector, to which the child in question will also return in a couple of years' time.

But for her Labour critics and teachers' representatives the best should be available in the state sector.

If it is not available, they argue, then the government has not achieved its stated goals on state education.

If it is available, then decisions to opt out must be being based on other factors, such as attempting to buy social privilege.

Ms Kelly says in her statement that "bringing up children in the public eye is never easy". Certainly not if you are a Labour minister and you decide to send one of your children to a private school.

She acknowledges that "some will disagree with my decision" but, in the end, she says, "we all face difficult choices as parents and I, like any mother, want to do the right thing for my son".