Boys at primary school perform 'significantly' better in English tests if they are taught in classes with fewer girls, a new study claims.

Research from Bristol University, which used data from every state school in England, found that as the proportion of girls rose, the results achieved by their male classmates fell. Steven Proud, who carried out the work, concluded it 'might be beneficial for boys to be educated in single-sex classes' in English.

He argued that girls tended to be ahead of boys in English, and so were more likely to answer questions, raise their hands and behave confidently in lessons. Boys studying alongside a large number of girls find it easier to 'hide in the background'.

'There is some thought that it could be down to boys free-riding in English classes,' said Proud, from the university's centre for market and public organisation. 'The more girls there are, the less they need to work. That is one supposition. Since girls perform considerably better in English, if there are more girls in the class, they are more likely to volunteer answers, so boys can hide in the background and it still appears the class is doing well.

'The other possibility is that there is some link between the sex of the teacher and how they focus their teaching. If a female teacher is teaching a lot of female pupils, they could focus their teaching towards girls and that could negatively effect the boys.'

In his working paper, Proud said the results implied 'that boys would benefit at all ages from being taught English in English schools with as small a proportion of girls as possible'.

Mick Brookes, general secretary of the National Association of Headteachers, said that boys who were a minority in a class could 'feel swamped by the high-attaining girls'.

'English is seen as a girly thing to do but that is wrong,' he said. 'What schools have to do is look at the subject matter in English and if it appears to have a feminine bias, then they need to look at that and see how it can be changed.'

Parents, meanwhile, called for teachers to be trained in dealing with classes where there were gender imbalances.

Margaret Morrissey, of the National Confederation of Parent Teacher Associations, said: 'I suspect that when there are fewer boys they are not going to feel as confident to put their hands up and are worried they will look silly if they get it wrong.

'There are lots of ways it can be dealt with and perhaps it is important to input this into teacher training.'

But some academics argued that more work needed to be done first. Alan Smithers, professor of education at the University of Buckingham, said he thought Proud had highlighted a 'fascinating pattern'. 'But we need to see what is producing it,' he said.

Smithers argued that only 15 per cent of educational performance was due to the nature of the school and factors such as gender balance. 'Most of it is attributable to other factors, such as ability differences or home background differences.

'It is very interesting but we need to delve more deeply. It would be wrong to rush to judgment.'