The Institute of Careers Guidance says a handful of former local authority staff have refused to go into schools which urged them to give favourable information about their range of options to potential sixth-formers.

A small number of schools were so anxious to retain prospective sixth-formers they kept careers information on other institutions in cupboards where pupils could not get hold of them.

Cathy Bereznicki, chief executive of ICG which represents around 4,000 careers officers, said the number of schools resorting to cloak-and-dagger tactics was low - but the disclosure lends credence to the latest report by the Office for Standards in Education.

It revealed that nearly a quarter of schools failed to resolve the tension between providing objective post-16 guidance and marketing sixth-form courses - a situation made worse following the incorporation of colleges according to Allister McGowan, chief executive of Hertfordshire Careers Service and ICG president.

Mrs Bereznicki said some schools applied “subtle pressure” to retain pupils such as telling parents that brightest pupils were expected to stay on until 18.

Ruth Gee, chief executive of the Association for Colleges, called for greater co-operation in the post-16 sector: “Collaboration is in everyone’s interests. It is in the students interest to receive the most relevant and comprehensive information available on the range of courses - nobody wants a disappointed student and well-informed students mean increased staying-on rates.”

But Anthony Barnes, president of the National Association of Careers and Guidance Teachers and an OFSTED inspector, said: “Nobody is squeaky-clean. Colleges like to blow the whistle and cry foul but they have generous marketing budgets and can go for the hard sell.”

He welcomed new inspection guidelines over schools’ even-handedness in careers guidance.