The party is urging the education secretary to step in after what it called the “disastrous handling” of results in Scotland where 125,000 marks – a quarter of the total – were lowered.
Some 15.2 per cent were cut in the poorest communities, compared with just 6.9 per cent in the most affluent areas, Labour said – because “prior attainment” in schools was used.
It showed “what can go wrong when computer algorithms drive students’ grades, and politicians wash their hands of responsibility”, said Kate Green, Labour’s shadow education secretary.
In a letter to education secretary Gavin Williamson, Ms Green wrote: “It’s imperative the government acts now to reassure worried students, teachers and parents.
“Young people deserve to have their hard work assessed on merit, but the system risks baking in inequality and doing most harm to students from disadvantaged backgrounds, those from ethnic minority groups and those with special educational needs and disabilities.”
Results are calculated using estimates made by teachers based on a pupil's performance over the school year but are then moderated.
Disappointed pupils in Scotland, handed lower grades than they achieved in prelim exams, are claiming they have suffered because they are from less affluent areas.
Opposition politicians have warned of a “deluge” of appeals and accused the Scottish Qualifications Authority of treating the professional judgement of teachers with “contempt”.
The row comes after Mr Williamson was accused, in June, of ignoring warnings that “unintentional bias” will penalise poorer and ethnic minority pupils this summer.
The Runnymede Trust told MPs it had written to him about the looming potential danger, but he had failed to respond.
In her letter, Ms Green alleged that “a generation of young Scots” had been failed, adding: “We cannot allow that to happen here in England next week.”
She urged Mr Williamson to explain “what protections the government is putting in place” to prevent a repeat south of the border, and explain the support on offer to pupils planning to make appeals.
College and university leaders might need to be more “flexible when making offers”, she suggested.
But Mr Williamson said: “No system could be a perfect substitute for real exams, but Ofqual [the exams regulator] has made every effort to ensure this summer’s grades are awarded fairly, setting out their working transparently for students, parents and teachers.
“We have put in place a package of support and guidance to minimise adverse impacts on students, including the chance to sit exams in the autumn.”