Maths experts are targeting girls in primary schools because they believe a "significant number" are slipping behind in the subject.
Some girls do not progress from basic methods, experts say
They say girls who perform well in national tests at seven can struggle later and do badly in tests at 11 because they rely on basic methods.
Experts from England's national numeracy strategy say they have spotted this trend in recent years.
Some local authorities are now trying new methods to help girls catch up.
Council-based maths consultants have been analysing national test results to identify schools where girls may be falling behind.
Tim Coulson, director of the national numeracy strategy, said: "Nationally, it seems there is a group of girls who are just getting by at seven using weak, inefficient and laborious strategies.
"They get the right answers, however, so are not picked out as struggling".
He said this group will not have understood other quicker ways of problem-solving so will have difficulty as the class moves on to bigger sums and will fall behind.
"They get to the juniors and questions get harder. They can no longer do them on their fingers and suddenly their mathematics world falls apart."
Boys, it is argued, are less likely to keep trying to use the old strategy for the bigger sums. They will use a new one or just get the sum wrong so the teacher will see more easily that they do not understand.
Mr Coulson said the under-achievement of girls in maths was not as great as the under-achievement of boys in literacy but still needed addressing.
Awareness of the maths gap had heightened in the past two years, he said.
In Berkshire, a dozen schools have been taking part in programmes designed to tackle the problem by asking children to talk in pairs or groups about how they would do different sums.
Similar efforts are being made in Lancashire, where 10 schools have been asked to come up with ideas to raise girls' achievement in maths.
Karen Sawyer, head teacher of Hungerford Primary in west Berkshire, told the Times Educational Supplement that test results had improved thanks to following this talking approach.
Last year, 82% of pupils at Hungerford reached the expected level 4 in maths at 11, compared with 70% in 2004.
"We thought girls didn't enjoy maths, but we found that they did but lacked confidence. They were not so keen to stretch themselves and played it safe," Ms Sawyer said.