Today's middle-class children are on track to be the first in more than a century to be materially less well off in adulthood than their parents, a government commission is expected to warn this week.

Leaked findings reveal the existence of a national trend not experienced since the early 20th century, with children from families with above-average incomes, as well as the most deprived, set to enjoy a worse standard of living when they grow up than their mothers and fathers.

The social mobility and child poverty commission, established by David Cameron, is expected to warn that government initiatives have all too often been aimed at the poorest 10%. Yet the inability to get on in life is a now a major and growing problem for middle-class children and this group is in dire need of attention, it is expected to report.

A Whitehall source said: "This will be controversial, but for the first time in over a century there is a real risk that the next generation of adults ends up worse off than today's generation. This is a problem for the children of parents with above-average incomes, not just a problem for those at the bottom. Many, many children face the prospect of having lower living standards than their parents."

The findings, to be laid before parliament on Thursday by education minister David Laws, will electrify the political debate over the so-called "squeezed middle", who have done so badly in the economic downturn.

Such is the expected political impact that the planned publication date for the commission's report was delayed so that it did not clash with the party conference season and become "a political football", according to one government source.

Among its conclusions, the commission is expected to say that those at particular risk are low-attaining children who are not poor enough to enjoy additional help from the system, but whose parents are not wealthy enough to insulate them from failure.

Pupils on free school meals benefit from an additional spend of £14,300 to improve their chances in life through the pupil premium. Yet nearly two-thirds of those who fail to attain an A to C grade in English and maths are from backgrounds not considered to be deprived. The cross-party commission, chaired by former Labour minister Alan Milburn and whose deputy is John Major's one-time education secretary Baroness Shephard, describes this group as the "missing piece in the jigsaw" of the government's education policies.

The commission is also expected to warn that children in the south-east but outside London are being let down by the system. Children in the capital on free school meals do 50% better in their GCSEs than those in other regions. A major cause of this geographical shift, the commission is expected to say, is that a higher proportion of high-quality heads and teachers live in disadvantaged areas in London than elsewhere, in part because of the so-called "London weighting" wage lift for those in the capital. Some of the weakest schools, it is set to point out, are located in bastions of middle England, such as Peterborough, west Berkshire, Herefordshire and satellite areas around London.

The commission is also expected to warn of a gathering "perfect storm" of graduate debt, lack of finance to buy homes and job insecurity that threatens middle-class children as they emerge from full-time education. Graduates will come out of university with up to £50,000 of debt to their name; the proportion of 25- to 34-year-olds who own their own homes has fallen from 60% to 40% in the past decade and is expected to plunge further; and the number of 18- to 24-year-olds unemployed for more than two years is at its highest since 1994.

The commission is expected to note that for the first time, a grandmother in her 80s can expect to enjoy higher living standards than someone in their 20s even if they are working, due to housing costs and poor wages.