It is a "national scandal" that poorer pupils are lagging up to a year behind their richer classmates in their schooling, Stephen Twigg will warn teachers on Tuesday.
The shadow education secretary is due to say that inequality in education is accepted too often, with youngsters condemned to "mediocrity" because it is assumed they cannot succeed.
In a speech to the Association of Teachers and Lecturers (ATL) annual conference in Manchester, Twigg will say that "double disadvantage" - coming from a poor background and attending a poor school - is a growing problem, and vow that a Labour government will make addressing the issue a top priority.
He will tell delegates that research has shown that over a school year, poor children who have very good teachers gain a year-and-a-half of learning, compared with half a year with a poor teacher.
"In other words, being a poor pupil in a poor classroom is the equivalent of being left a year behind.
"This is a national scandal.
"I know there are inequalities in our health system, but if poorer patients were left to linger on waiting lists for an extra year there would be a huge outcry.
"But too often in education we accept inequality - condemning certain children to mediocrity because we assume that they cannot achieve success.
"This is one of the biggest barriers to social mobility today.
"So one of the top priorities of a future Labour government will be to address these areas of 'double disadvantage'. The double whammy of a poor background and a poor school - creating a cycle of poverty that can exclude generation after generation."