Approximately 40 per cent of Welsh schoolchildren are below the standard expected of them when it comes to reading.
Welsh school inspection body Estyn said writing skills are also poor in many schools, with male pupils struggling to keep up with their female counterparts.
Ann Keane, the organisation's chief inspector, told BBC Radio Wales: "The issue is not that literacy and numeracy is the province just of maths, English or Welsh lessons, but that it should be the basis for how teachers teach all subjects in the curriculum.
"These are the areas that we would like teachers to work harder at and these are the areas that we would like to see the professional learning communities of teachers discussing how to improve."
A report published by the organisation noted that improvement has been "slow" in recent years.
Speaking this week, John Coe, secretary of the National Association of Primary Education, said there is a direct link between reading ability and social background.