The proportion of 18 to 24-year-olds in England not in employment, education or training (Neet) has risen to 18.4%, official figures suggest.
The figure from the Department for Education is the highest for the second quarter since 2006, and is up from 16.3% last year.
Nearly a million (979,000) 16 to 24-year-olds were Neet between April and June this year, the figures show.
The government said it was boosting apprenticeships and 16 to 18 education.
Neets figures fluctuate during the academic year, peaking in the third quarter as school, college and university courses end.
But this year's 18 to 24 figures are the highest for the second quarter since comparable data was first published in 2006; the previous was 17.6% in 2008.
The figure is even higher for 19 to 24-year-olds, with 19.1% considered Neet.
The data is published for England only by the Department for Education, derived from the quarterly Labour Force Survey.
The proportion of young people in England aged 16 to 18 who are considered Neet has continued to fall, however, amid a government drive to encourage more young people to stay on in education or training.
It dropped to 9.8% in the quarter to June, down from 10.2% at the same point in 2010, and 11.9% the previous year.
The figures come as record numbers - an estimated 220,000 - are expected to fail to get UK university places.
And last week, figures from the Office for National Statistics showed that youth unemployment in the UK - the proportion of 16 to 24-year-olds not in work - had also risen, from 20% to 20.2% in the quarter to June.
The Prince's Trust youth charity said it was "deeply concerned" that the Neets figures were rising again.
"We know that unemployment can have a brutal impact on young people, with thousands suffering from mental health problems, feelings of self-loathing and panic attacks," a spokesman said.
A government spokesman said the number of young people not in education, employment or training "has been too high for too long".
The fall in the number of 16 to 18-year-old Neets was an encouraging sign, and the government would set out a strategy for post-16 participation in education later this year, the spokesman said.
The government will have supported 250,000 more adult apprenticeship places than under Labour's plans and has launched a Work Programme offering personalised support and training to help unemployed young people, the spokesman added.
However, shadow education secretary Andy Burnham said the figures showed the government was "being far too complacent" and risked "leaving the next generation behind".
"By scrapping Labour's guarantee of an apprenticeship place for young people who want one, scrapping the Education Maintenance Allowance [grant for low-income 16-19 year-old students] and cutting careers services, this government is making it harder for young people to get on," he said.
"For the first time there is a risk that the next generation will do worse than the last," he added.