The achievement gap between pupils entitled to free school meals and their counterparts at the age of 15 has reached an all-time high, new figures show.

In 2006, the gap in the percentage of key stage 4 pupils gaining five A*-C GCSEs or equivalent qualifications, including English or Welsh and maths, was 29.5 percentage points - last year that had grown to 34 percentage points.

Known as the level 2 threshold, the target is one of the six key indicators the Assembly government is using to measure the progress of its Child Poverty Strategy, which was launched last month.

Although the performance of pupils eligible for free school meals is lower than their counterparts in all key stages, the gap has narrowed at KS1 and KS2 and remained stable at KS3.

Free school meal entitlement has often been used to measure the socio-economic conditions of a school’s population, but critics point out that while there is a link between entitlement and achievement, many other factors affect results.

David Egan, professor of education at the University of Wales Institute, Cardiff, said the “strong and enduring” link between poverty and low attainment is one of the major weaknesses of Wales’s education system.

“It’s disappointing that the gap is widening given everything that has been done in recent years, particularly in relation to 14-19 policy,” he said.

“It’s another piece of evidence that shows there is a major issue that has to be tackled, and we have to step up our efforts.”

Education minister Leighton Andrews recently outlined a series of actions to improve every aspect of the education system by raising literacy and numeracy standards and narrowing the equity gap.

An Assembly government spokesman said: “We will use every opportunity and policy lever available to break these links and ensure that children and young people from disadvantaged backgrounds do just as well as their better off peers.”