There has been a "shameful" rise in the number of young people in England leaving school without five good GCSEs or equivalent technical qualifications, the children's commissioner has said.
Research found a 28% increase since 2015, with about 100,000 pupils a year affected - almost one in five
Anne Longfield said the impact on the poorest families was most marked.
The government said her report did not provide the full picture and standards were improving "across the board".
But Ms Longfield urged ministers to focus on the "absolutely devastating" impact on the children "who are being left behind".
Speaking on BBC Breakfast, she said: "I know there has been a huge amount of effort into attainment but I think a lot of that has been on raising grades at the higher end.
"As so often is the case, children who are on free school meals - the poorest children - and children with special educational needs fare the worse in the situation."
The research for the children's commissioner was based on analysis of official statistics and found 98,779 (18%) of pupils in England in 2018 had failed to gain five GCSEs at grade C or higher or the equivalent technical qualifications. Of pupils who qualified for free school meals, about 28,225 (37%) did not achieve this level of attainment.
"These are children who will have spent 15 years in compulsory education, often having more than £100,000 of public money spent on their education and yet leave the education system without basic benchmark qualifications," the report for the children's commissioner says.
"Many will not be able to begin an apprenticeship, start technical courses or enter some workplaces because they cannot meet the basic entry requirements."
Shadow education secretary Angela Rayner described the findings as "shocking", blaming "brutal cuts on education and support for families and children" under the Conservatives.
Paul Whiteman from the National Association of Head Teachers cited the "impact of real-terms cuts to school funding" in recent years.
'Range of options'
Figures show that 33% of teenagers left school in 2005 without five GCSEs. By 2010 this had fallen to 21%, and by 2015 to 14%.
The report suggests the increase in pupils leaving without qualifications in the three years after 2015 has been partly caused by some schools stopping offering vocational alternatives to GCSEs. The school leaving age was also raised from 17 to 18 in 2015, meaning that some pupils who would have previously left secondary education were staying on.
The Department for Education questioned the accuracy of the research, saying it made comparisons with qualifications that had been removed from performance tables.
A spokesperson highlighted improvements in pass rates for English and maths GCSEs and said the gap between disadvantaged pupils and their peers had narrowed since 2011.
They added: "We are working to dramatically improve the rigour, quality and standard of qualifications across the board, and have already done so with GCSEs. These reformed qualifications will help young people achieve the skills they need to get on in life."
Anna Round, a senior research fellow at the Institute for Public Policy Research think tank, said it was not possible to determine the reasons for the rise - although changes to qualifications, school leaving ages and a focus of policy on attainment probably have played a part, as well as resources.
But she said Ms Longfield was right to call for action.
"Education shouldn't just separate young people into passes or fails", Ms Round said, adding: "That means making sure schools can offer a range of options to match the diverse interests and needs of their students."