Government figures show only 15% of white working class boys in England got five good GCSEs including maths and English last year.
GCSE data on pupils receiving free school meals has been released
Among white boys from more affluent homes - 45% achieved that level of qualification.
Poorer pupils from Indian and Chinese backgrounds fared much better - with 36% and 52% making that grade respectively.
Ministers say they are narrowing the gap between affluent and poorer pupils.
The national average for all pupils in England achieving five good GCSEs including English and maths (A* to C) was 46% last year.
Liberal Democrat spokesman David Laws said: "We should be ashamed to live in a country where there is such a huge gap between rich and poor children.
"To have 85% of white boys from poor families failing to achieve five good GCSEs including English and maths is truly shocking.
"The government has failed to tackle the chasm that exists between the opportunities of most of the poorest and the richest in our society.
"We need a massive targeted increase in funding for deprived young people, to allow more catch-up classes and additional support to give every child a chance."
Shadow children’s secretary Michael Gove said: “The government's failure to improve standards in education has hit the poorest hardest. We need a school system that allows bright children to succeed regardless of their economic background.
Closing the attainment gap in education remains a top priority, and we have made encouraging recent progress
Jim Knight, Schools minister
“We can only achieve this by focusing on the basics like getting all children reading after two years of primary school. Instead we still have a system where the achievement gap between rich and poor pupils grows as they progress through their school careers.”
GSCE performance data released by the government in November did not include details of pupils receiving free school meals - an indicator of poverty.
Those details have now been published.
Schools Minister Jim Knight said: “Closing the attainment gap in education remains a top priority, and we have made encouraging recent progress.
"There has been good news on our efforts to address social mobility, with pupils eligible for free school meals improving faster than average.
"Between 2003 and 2007, pupils eligible for free school meals who achieved 5 good GCSEs rose 11.1 percentage points from 24.4% to 35.5%. For non-free school meals pupils, the increase was 7.6 percentage points, from 55.2% to 62.8%.
"Alongside pupils on free school meals, previously disadvantaged groups are also doing better. Over the last four years, black pupils have made the biggest improvement, at almost twice the national average."
Policies had been introduced to try to help underachieving boys, he said.