The proportion of schoolchildren not doing homework every night has risen since the 1990s, a survey suggests.
The government produced homework guidelines in 1998
The Schools Health Education Unit (SHEU) found it had risen to two-fifths last year from one-fifth in 1997/98.
But the Department for Education and Skills said homework remained an "essential part" of education.
SHEU researchers asked 17,743 pupils aged 10 to 15 in England, Scotland and Wales whether they had done any school work the previous night.
The SHEU's research manager, David Regis, said: "I would have thought that, as schools are so much more focused on exam results and coursework, that pupils would be doing more homework.
"I suppose some of it is to do with what we call homework.
"If a teacher asked children to come back next week having found out as much as they know about tigers, they might not think of it as homework.
"I would call it that, but the kids might think of it as fun."
In 1998, the then education secretary, David Blunkett, produced recommendations that secondary pupils in England and Wales should do about 90 minutes of homework a night.
The SHEU survey found that girls aged 12 to 13 were the most likely to do so. Some 64% said they had done homework the previous night.
Boys aged 14 to 15 had the lowest rate - 56%.
Shadow schools minister Nick Gibb said: "Such a dramatic fall in the proportion of children doing homework is a very serious indictment of standards in our schools.
"Ofsted should be asked, as part of its routine inspection process, specifically to report on the homework policy of the school and the extent to which the policy is being implemented in practice."
A DfES spokesman said: "Homework is an essential part of a good education.
"A good, well-organised homework programme helps children and young people to develop the skills and attitudes they will need for successful, independent life-long learning."
The SHEU's annual Young People survey's other findings included that 22% of 14 to 15-year-old boys were "fairly sure" or "certain" that friends carried weapons for protection when going out - a figure similar to recent years.
Some 44% of girls aged 10 to 11 reported having feared going to school because of bullying.
Mr Regis said: "Bullying figures have remained pretty constant over recent years, even though there are so many initiatives aimed at stopping it.
"Perhaps this is because there is more awareness, leading to more likelihood of reporting it. It is still a worry, though."
Up to 41% of 14 to 15-year-olds had consumed alcohol in the previous week, while 57% were "fairly sure" they knew a drug user.
Some 21% of males and 15% of females of this age were overweight.
The SHEU, based at Exeter University, spoke to children and staff in 310 primary and secondary schools.