Ministers have been told to stop threatening Oxford University with financial penalties for not admitting more pupils from poorer backgrounds.

Michael Beloff, the president of Trinity College, said the university owed its international standing to its insistence that academic merit was the only criterion for admission.

"To alter our standards in pursuit of social or political rather than educational objectives would be a betrayal of what the university is for," he told the annual meeting of the Headmasters' and Headmistresses' Conference.