The proportion achieving at least five grades A*-C this summer went up one percentage point to 52 per cent. In England, it rose by two percentage points - the largest increase in a decade - to 55.7 per cent, according to statistics published last week.

But Welsh heads say the gap may be artificial as English schools enter pupils for exams that get higher scores in tables.

Welsh students matched English teenagers’ GCSE performance from 1998 to 2001. But since then results have improved less than in England, allowing teenagers across the border to pull ahead. Unions and educationists suggest several reasons, including underfunding of secondary schools and the slower take-up of vocational qualifications in Wales.

But Brian Rowlands, secretary of the Secondary Heads Association Cymru, said the latest English GCSE figures highlighted how league tables could be manipulated - making comparisons with Wales inappropriate. Some schools have achieved big rises in pass rates by entering pupils for vocational qualifications, worth up to four GCSE passes. At the same time, results for five subjects, including English and maths, have worsened in some cases, he noted.

“Schools in England enter pupils for exams that count in league tables because that’s how they are judged. There is a rapid rise in GNVQ entries, and there’s nothing wrong with that. But in many ways that artificially raises the results,” he said.

He predicted the pattern of entries would change again in 2006, when the Westminster government sets targets for five good GCSE passes including English and maths. That pass rate this summer was 44 per cent.

Mr Rowlands also pointed to funding concerns - reinforced in part by education minister Jane Davidson, who told heads earlier this month that Wales’s primary sector is relatively better funded than its secondary schools.

The SHA Cymru claims secondary schools in Wales receive around pound;150 less per pupil than in England. A committee set up by National Assembly members to investigate school funding holds its second meeting next week.

In Wales, the Assembly government sets targets for five A*-C grades, but also for a “core subject indicator” of EnglishWelsh, maths and science.

This summer, 38 per cent of pupils passed all three GCSE subjects at grade C or above, the same as last year.

An Assembly government spokesperson said there could be several reasons for the difference in results. He added: “In Wales, we prefer to look at year-on-year progress and the results for 2004-05 show that we are maintaining our very high standards - since 1997-98 results for five or more GCSEs grade A*-C have risen six percentage points.”

David Reynolds, professor of education at Plymouth university, acknowledged that England’s improvement could be down to tactics. But he added: “It may also be that some of the wide range of programmes that the English government has thrown at its schools have actually stuck at last.”

Programmes such as excellence in cities and education action zones have provided additional resources for curriculum development and improving learning and attendance in England.

Inspection agency Estyn has praised the Assembly government’s own KS3 strategy (see opposite). But performance figures comparing results at similar schools and against national standards have only recently become available in Wales - whereas schools in England have been receiving such data for years.

GCSE gradeS 16