The BBC's Sue Littlemore: More parents are swopping schooling for the Costa del Sol

The General Secretary of the National Association of Head Teachers, David Hart, said the trend was associated with family break-up. Children whose parents were divorced were increasingly being taken on two main holidays.

It is known already that parents are increasingly removing children from lessons to take advantage of cheap out-of-season offers from travel companies.

Travel firms charge more during school holidays

The Education Secretary, David Blunkett, is asking ministers to talk to the Association of British Travel Agents about removing those incentives - although the industry is likely to argue that it is simply a matter of supply and demand.

But another new factor, the headteachers' association believes, is the extension of the working week. Parents working long hours, seven days a week, are taking holidays when they can.

One secondary school recorded more than 2,000 pupil half-days lost to holidays in term-time in the last school year, Mr Hart claimed. The impact on children's education could be significant, he said.

Law 'out of date'

"The present law permits pupils to have up to 10 days authorised absence per school year, but parents are increasingly taking advantage by treating this as a right.

"It is patently wrong that parents cannot, or will not, find time for family holidays within the existing 14-week envelope."

The government is recommending that home-school contracts, which each school must now issue, should remind parents of the law regarding school attendance. But Mr Hart does not think this is tough enough.

Nice - but naughty, says the NAHT

"The present law is out of date and totally at odds with the government's drive to raise standards. It should be changed because pupils can ill-afford to lose any time during the school year," he said.

Holidays were often "stretched" by parents starting their Christmas break a week early, or added an extra week either side of the spring or summer half-term breaks or at Easter.

In areas with large numbers of ethnic minority parents, schools reported that some children go to Pakistan, India or the Caribbean from November or December to January or even as late as March, for family reasons.

Last May, the government began a campaign with the Bangladesh High Commission in London to discourage this sort of thing.