Almost a third of 11-year-olds could be leaving primary school this summer without a good grasp of reading, writing and maths, it was suggested today.
The prediction comes the day before the Government publishes new figures on the proportion of primary school pupils reaching the level expected of them in the basics.
Last year, just 65% achieved the standard expected for their age (Level 4) in reading, writing and maths combined, meaning that 35%, over a third, missed out.
Professor Alan Smithers, director of the Centre for Education and Employment Research at Buckingham University, said the figure reaching the level is likely to rise this year, as schools will have been focusing on this measure.
Professor Smithers said: "I would think that since the spotlight focused on that result that schools will have put extra effort in there.
"I think that it might be getting up to 70%.
"But that still looks as though 30% are leaving without reaching the expected level."
Last year was the first time the Government had specifically highlighted this particular measure of pupil performance, Professor Smithers said.
"I would expect schools to have put more effort into ensuring that those that can achieve all three can do so, previously it was all about individual performance in English and maths."
But he warned that bright children, or those that struggle the most, could be affected if schools are focusing on those on the borderline of reaching Level 4.
"People who need to be stretched more, and those that are really struggling at the other end of the scale. They could be ignored," he warned.
This could include youngsters who have special educational needs, or have just arrived in the UK. Some children are also good with either words, or numbers, but have difficulty with the other, Professor Smithers added.
Christine Blower, general secretary of the National Union of Teachers, said: "When we ask people they have said there's always a problem that when a school is under pressure what they do is deal with the children that are borderline."
National curriculum tests, known as "Sats" are taken by pupils in their final year of primary school, and have been fiercely opposed by teaching unions, with a boycott held last year.
According to last summer's results, 80% of pupils reached Level 4 in English, and 79% reached it in maths.
Ministers announced plans last month, following an independent review, to scrap the creative writing paper from 2013.
The writing paper, which forms part of the overall writing test, has long been seen as the most controversial, with teachers arguing it is hard to mark because answers are open to interpretation.
Pupils will sit a beefed-up test in spelling, grammar and punctuation in the future.