The number of pupils eligible for free school meals has risen sharply in the past three years, prompting fears that the attainment gap for poor children could grow.
Provisional figures from the latest school census in Wales reveal that 20.7 per cent of primary pupils and 17.4 per cent of secondary students are now eligible for free meals.
This is up from 17.9 and 15.6 per cent respectively in 2008.
There has historically been a strong link between poverty and low attainment, and experts have warned that the effect could get stronger if urgent action is not taken.
The figures came as a separate report from Estyn said schools must get better at identifying and supporting learners from disadvantaged backgrounds.
At all key stages in Wales, pupils eligible for free school meals perform significantly worse than their peers, and the performance gap is particularly large in secondary schools.
After a peak in the mid-1990s, there had been a declining trend in the percentage of pupils eligible for free school meals.
Education minister Leighton Andrews has made tackling the link between poverty and attainment one of his three key priorities, but the census figures suggest the task may become more difficult.
David Egan, professor of education at the University of Wales Institute, Cardiff, said the rise in free school meals was “inevitable” because of the recession.
“The challenge we face is considerable and it’s becoming greater,” he said. “There is a danger that the effect on results will get stronger, but disadvantage should not be an excuse or an alibi for low attainment.
“We are a country that has been blighted by this link, but we shouldn’t accept it is the case. There are pupils and schools that buck the trend.”
David Reynolds, professor of education at Southampton University and an adviser to the Welsh Government, said the rise would have an effect on education, but that it would not be significant.
“This is a serious situation, but it’s not serious enough to derail what the Government is trying to achieve,” he said.
Professor David Hopkins, a senior policy adviser to the Department for Education and Skills, said the situation could be overcome.
“It is difficult and challenging but we can do it. Some of our best schools are working in the most challenging circumstances.
“Speed is of the essence. You can get a significant shift in pupil performance in a couple of terms. Urgency on the ground and quality of implementation is vital.”
Estyn’s report said being eligible for free school meals does not mean pupils are destined to underachieve.
Those from relatively poor backgrounds can and do achieve at the highest level and schools serving poor neighbourhoods can outperform schools with more affluent catchment areas, it said.
However, it said that schools must get better at monitoring the progress of all disadvantaged pupils, as most do not have good enough systems to identify individual needs.
Chief inspector Ann Keane said: “Research shows that socio-economic disadvantage is the biggest obstacle to achieving a good education.
“Learners from poorer families are more likely to attain at lower levels. It is often harder for them to gain high-skilled employment or continue their education after school.”
The report said a few schools have raised the achievement of their disadvantaged pupils through effective skills-based teaching and individual support such as mentoring and help with basic skills.
A Welsh Government spokesman said tackling the link between poverty and attainment remains a priority.
“Figures showing an increase in pupils eligible for free school meals could be a result of more people claiming jobseekers’ allowance in Wales, which is one of the criteria for eligibility,” he said.
“With this in mind, it is even more important that we work with local authorities and schools to break this link.”
