Schools with 20 per cent or more of pupils from poor backgrounds see lower attainment for all children, a global study has found.

Research by Lancaster University found that attainment for all pupils in a UK school falls if as few as one in five students are classed as disadvantaged.

The study, which compared educational attainment across nine countries, concluded that pupils in the UK are more affected by the social background they come from than any other factor.

Researchers said the figures showed that the "tipping point" at which a school became a "sink school" which holds its pupils back was based on "quite a low proportion of disadvantaged students going to that school".

"As soon as you've got a lot of disadvantaged students in a school, that is going to cause a drop-off in the performance of a randomly picked student from that schools," said Geraint Johnes, a professor of economics at Lancaster University Management School, and co-author of the study, published in the European Journal of Operational Research.

Academics studied 15 years of maths scores by pupils taking the programme for international student assessment (PISA), a series of standardised tests for 15 year olds in reading, mathematics and science.