Nearly one in five said they were regularly surprised by the difficulty of the work their children took home to complete.
And 69 per cent said they would spend more time helping with their children's homework if they were more confident in their own abilities.
The findings are based on a survey for the Government of 2,000 parents with children aged 15 and under in April.
Margaret Morrissey, from the National Confederation of Parent Teacher Associations, said: "Parents' biggest problem is not that they can't find the answer, but not understanding how things are being taught. And why should they? Every time parents start to understand the curriculum the Government changes it."
Phil Hope, skills minister, said there were hundreds of free courses around the country to help adults brush-up their maths and English.
"As a dad I'm fully aware of what a difference it can make to be able to help your children, so I call on all parents to think about whether they could benefit from improving their skills a little more," he said.
More than nine out of 10 parents surveyed agreed that helping children at home makes a difference to their academic achievements at school.
Over half - 53 per cent - said they helped their children with homework "every day".
According to government figures, 5.2 million adults lack the English skills expected of a 14-year-old. And 14.9m would be unable to match a typical 14-year-old at maths.