The Children's Plan will focus on improving the quality rather than increasing the quantity of teaching the basics, according to the head of the government's review of the primary curriculum.
Last week, ministers unveiled the 10-year programme to raise standards and improve pupils' lives.
They set a target of getting 90 per cent of 11-year-olds up to acceptable levels in English and maths by 2020, which they hope to achieve with a slimmed-down curriculum that will create more time for the two subjects.
But Sir Jim Rose told The TES that ensuring quality and progress in the subjects was more important than dictating curriculum time.
"What I want to do is look at the relationship between timetabled English work - which has been very much the literacy hour - and the rest of the curriculum, where English is taught either explicitly or implicitly," he said. "I will be identifying first and foremost where (in the curriculum) time is spent on teaching English and maths. We need to know more about what really happens before making judgments about time allocations."
Sir Jim accepted the invitation to lead the 12-month review a week before it was formally announced. He will report in March 2009, and the new curriculum will be in place by 2011.
The brief set out in the Children's Plan was to set aside "more time for the basics, so children achieve a good grounding in reading, writing and maths; greater flexibility for other subjects; time for primary school children to learn a modern foreign language; and a smoother transition from play-based learning ... particularly to help summer-born children, who can be at a disadvantage when they enter primary school".
Researchers from Manchester University's Centre for Formative Assessment Studies looked into primary curriculum time allocations and found that junior children spent almost half their week in English and maths lessons in 2006. But the figures also reveal that most English is being taught through other subjects, such as history.
Sir Jim said: "Still the best way of teaching, I think, is to plan through the subjects. Time is finite and primary schools can only do so much."
Teachers writing about the primary review on The TES website were keen for it to be carried out, but were divided on priorities: circle time and design and technology came in for criticism, as did learning about genres in English. There was support for making time for music, PE and handwriting.
This will be the third review of the national curriculum in primaries since it was introduced in 1988.
In 1993, Lord Dearing slimmed down the original version. His final report recommended that primaries teach English for 180 hours a year - roughly an hour a day in key stage 1 - and that they spend a further hour each week teaching English through other subjects. In key stage 2, he recommended 162 hours of literacy lessons a year and a further hour each fortnight on English through other subjects.
In 1998, the national literacy strategy was introduced and literacy hour became standard in almost all primaries. Daily maths lessons followed in 1999. In 2000, the Qualifications and Curriculum Authority altered the curriculum to ensure consistency with the literacy and numeracy strategies. More than a third of content was cut from non-core subjects, and modern foreign languages were encouraged.