The Government wants 85 per cent of 11-year-olds to reach the expected level for their age by 2004. Last year only 73 per cent achieved level 4.
At the core of the National Numeracy Strategy is the five-day course, which aims to improve teachers' subject knowledge.
But the Office for Standards in Education and researchers from the University of Toronto have said that what is being learned in training is not necessarily being implemented in the classroom.
Now strategy officials are recommending that local education authorities provide three days of out-of-school theory and two days of school-based training.
Funding will be provided for the full five days, enabling supply cover to continue while the teacher is doing school-based training.
Tim Coulson, the strategy's director, said: "We are not saying that all local education authorities should immediately stop doing the old five-day course and use the new one.
"This model will still have five days of funding, so a supply teacher will be in school for five days. But the teacher will be in school and can take a small group of children and have the luxury of focusing on them.
"By having gaps between out-of-school training days, the teachers can try out what they have learned with their pupils."
Some authorities are already offering more in-school support.
Nottinghamshire, where maths results in Year 6 rose seven percentage points between 1999 and 2002, already provides in-school training.
Primary strategy manager Peter Eardley said: "Many teachers need to be able to see it work in their classroom. Our heads value our training but they said what they need is this implementation time.
"Life in school is such hard work that a lot of good training simply gets lost. Teachers have been out of school and then come back to find reports, parents' evenings and 101 other things to do."