Children could be forced to resit their final year at primary school or go to summer school if their basic skills are not up to scratch under plans being considered by the Tories.
Shadow Children Schools and Families Secretary Michael Gove stressed making 11-year-olds stay back rather than go on to secondary schools would be "very much a backstop".
But he added: "We can't have children going from primary school into secondary school without the skills necessary to make the most of what they are going to be taught in secondary schools. What's the benefit of sending them to secondary school if they can't benefit from what they will be taught there?"
The proposals for remedial summer schools or staying back a year will be contained in a policy document on public services to be published by the Conservatives next week.
They are not formal party policy but will be considered for inclusion in the next Tory manifesto.
Leader David Cameron said the radical move could form part of a "genuine schools" revolution to drive up standards of literacy and numeracy, and improve discipline.
In an article for the Sunday Telegraph, the Conservative leader also backed calls for an "advantage premium" which would see headteachers' budgets boosted for taking youngsters from disadvantaged backgrounds.
Schools could get up to £6,000 extra for each pupil they took, making them more attractive propositions for headteachers worried about bringing in potentially disruptive elements.
Mr Cameron made his grab for the education agenda as Schools Secretary Ed Balls admitted Labour's progress on improving education appeared to have been tailing off during the past year. Mr Balls is writing to all headteachers today marking the start of the new school year, asking them to redouble efforts to enforce discipline and ensure children get basic skills.
"I'm going to be writing to all headteachers tomorrow with a letter to welcome them back to school, to say congratulations on the results, which are a steady improvement, but also to say it's not enough," he said.
"We aren't delivering for every child, we must give a world class education to every child in our country. It's economically necessary. It's also the right thing to do from the point of view of fairness.
"And therefore my message to teachers is I'm going to back you to do more to support the personal development of every child, more work to catch up if the kids are falling behind in maths or in English, more personal attention on each individual child's progress - back to basics."
In his article, Mr Cameron repeated his pledge to put "rocket boosters" under the Government's controversial City Academies scheme - a pet project of Tony Blair's which some believe will be scaled down by Prime Minister Gordon Brown. He said there should be a "bonfire of controls" to free teachers from bureaucracy and targets which make them "glorified form fillers".
Under a Tory administration there would be no more closure of special needs schools, and "zero tolerance" of badly-behaved pupils.
Mr Cameron has already insisted schools should have the final say over whether youngsters are expelled, rather than local education authorities. His comments came ahead of the launch of a report from his party's public services policy group, chaired by former Cabinet minister Stephen Dorrell.
The review will suggest that the worst performers in year six are made to either catch up at summer classes or repeat the whole academic year.
Mr Cameron prom-ised to "look carefully" at the measure, already used in the US and some European countries.
The proposals received a cautious welcome from former Education Secretary David Blunkett who said they shouldn't be ruled out, but Schools Minister Jim Knight said: "Like the old 11-plus, proposals for what the Tories have called a remedial year would stigmatise the very children who need extra help.
"They would also increase class sizes and make it impossible for teachers and parents to plan ahead.
"Instead of impractical gimmicks that haven't been thought through, we need policy that works."
And Liberal Democrat children, schools, and families spokesman David Laws said: "It would be far more sensible to earmark additional funding for measures such as extended school hours, more teaching on Saturdays and smaller primary class sizes to prevent poor performing students falling behind.