Figures show 70% of school leavers in England do not work out regularly
The majority of 19-year-olds in England do not take regular exercise once they have left school, according to figures obtained by the Liberal Democrats.
Only 30% of 19-year-olds participate in at least 30 minutes of sport, of at least moderate intensity, three times a week or more.
And just 20% of sixth formers reach the target of three hours of sport a week.
The number of 16 to 18-year-olds doing sport at least once a month has been in decline over the past four years.
The statistics - provided by Sport England and the Department for Culture, Media and Sport - were obtained by the Liberal Democrats in response to a parliamentary question.
The figures show 37% of 16-year-olds, 35% of 17-year-olds and 31% of 18-year-olds take part in at least 30 minutes of sport, of at least moderate intensity, at least three times a week.
There has also been a steady decline in the percentage of teenagers participating in at least one active sport a month.
In 2005-06, 83% of 16-year-olds were active once a month, but this fell to 78% in 2008-09.
Liberal Democrat sport spokesman Don Foster said: "Ministers' claims over successes in school sport mask the fact that large numbers of children still aren't doing enough exercise.
"Despite the government pumping billions of pounds into school sport, many older children are doing very little exercise and dropping out completely once they leave.
"It's ludicrous for ministers to be making commitments to raise the time children spend playing sport to five hours a week when some are doing less than half an hour a week.
"With the country threatened by an obesity time-bomb, it is vital that the government gets the basics right before making grand promises."