Boys’ schools are increasingly admitting female pupils to boost their image as they improve their exam results and send them shooting up league tables, according to a leading headmistress.

However, she warned that this could have a negative effect on the girls’ education.

Nicole Chapman, head of the state-funded Chelmsford County High School for Girls, argued that a boy-free zone was the most productive setting for many girls, as they could ‘totally be themselves’.

She told The Daily Telegraph: ‘If girls are in a single-sex environment they can totally be themselves. They don’t have to pander to anything to do with being a girl. If they love to do really sporty things, if they love their maths, science or IT, no boy will push them out of the way.

‘In the subjects that are traditionally seen as “male”, like maths and science, girls in girls’ schools perform better and are more likely to be encouraged than in mixed environments.’

She added: ‘Girls perform well so it adds good results to their league tables.’

Ms Chapman sits on the steering committee of the Association of Maintained Girls’ Schools, and her comments came on the eve of the organisation’s annual conference in London.

Admitting girls at sixth-form level is a standard policy for several high-profile boys’ schools, such as Magdalen College School in Oxford and London’s Westminster and King’s College schools.

Aside from PR for the school, Ms Chapman claims many boys’ schools admit girls because they are a calming influence on sometimes ‘rough’ male pupils.

Many girls, meanwhile, go to boys' schools for romantic reasons, she claimed.

There are 400 boys’ schools in England and Wales compared to 620 girls’ schools, with the overall number declining in the past 25 years.

Chris Keates, General Secretary of teachers' union the NASUWT, told MailOnline: 'The NASUWT is not surprised that stories are emerging about admissions into schools. The Government has fostered a culture of competition between schools and removed the mechanisms which were in place to monitor schools admissions.

'This is just another example of the market driven free-for-all the Coalition Government is creating in education.'