The Government cannot guarantee to meet its targets for recruiting teachers, Estelle Morris, the Secretary of State for Education and Skills, conceded.
Ms Morris told a conference in London, organised by the public services union Unison, that at least 10 per cent of all graduates for the next 15 years would have to be recruited for all teaching posts to be filled.
Her comments were seized on by teachers' leaders as the most candid admission yet by a minister that schools could face major staffing shortages for years to come.
Up to 45 per cent of today's teachers – nearly 200,000 – were aged between 45 and 60 and would be leaving the profession over the next 15 years. Ms Morris said: "They are all my age and it worries me like heck."
The Government was meeting its target at the moment but the minister said: "We need to recruit 10 per cent of the nation's graduates – and 40 per cent of maths graduates – year on year on year. I can't promise that will continue."
She called for a debate with headteachers over whether they should be spending extra Government funds on creating more teaching posts – instead of taking on more auxiliaries to help out in the classroom.
"I am not always sure they should be using the extra money we give them merely to create more teaching posts in schools," she said.
Schools were now employing 7,000 more teachers than in 1997 but – because of the creation of extra posts – there were still major staffing shortages.
"Heads are used to managing shrinking budgets, which they do very well. I am not sure they are managing increased budgets so well," said Ms Morris. Instead, they should be spending the money on an army of "classroom assistants, bursars, para-auxiliaries and technicians".
There needed to be a debate over the education service's "capacity to deliver world-class services" and classroom assistants needed to be given extra responsibilities such as invigilating exams or taking over lessons where work had been prepared, Ms Morris said.
Headteachers' leaders last night warned of the danger of "defeatism" and opting for classroom assistants as a "second best" to teachers.
John Dunford, general secretary of the Secondary Heads Association, said: "This is a stark reminder of the size of the task. It also highlights the importance of getting things right on teachers' pay and workload and pupil behaviour. We need to make teaching an attractive enough profession so those graduates will want to join it."
He rejected criticism of heads for creating teaching posts: "We're still at the end of a period of worsening pupil-teacher ratios in secondary schools, when heads are trying to get their teaching numbers up to a reasonable level. When they have done that, they will be looking to recruit extra support staff," he said.
Doug McAvoy, general secretary of the National Union of Teachers, said: "This is at least an admission by the Government that there is a fundamental problem. It will give hope to teachers because for a long time ministers have tried to portray the situation as only a small problem in one part of the country or in particular subject areas."
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies