A headteaching qualification which is currently required to take charge of state schools in England will no longer be compulsory, the Government says.
Ministers plan to make the National Professional Qualification for Headship (NPQH) optional from next year to give schools more choice, the Department for Education said.
A department spokesman said the move was designed to make the NPQH the "pinnacle of headteachers' qualifications" that would be "seen in the same light as an MBA".
But union leaders questioned the decision, calling it a "backward step".
At the same time, the Government will make it harder for people to study for the qualification, which will be made "more demanding".
The revised qualification, which ministers will introduce next September, will put more emphasis on dealing with pupils' behaviour. Other compulsory elements will include budget management and improving other teachers' performance.
The new NPQH will also require trainee headteachers to spend more time on school placements before they qualify, with the minimum length increasing from five to nine days.
The qualification was first introduced in 1997 to give teachers keen to become heads the skills they needed. It became mandatory for all first-time headteachers in the maintained sector in 2009. About 58% of current state headteachers have the qualification.
Schools minister Nick Gibb said: "The highest-performing education systems are those where government knows when to step back and let heads get on with running their schools."
But Mary Bousted, general secretary of the Association of Teachers and Lecturers, said: "We're not convinced of the need for such a radical overhaul of the qualification for headship. Headteachers should be focused on leading learning and teaching. We are concerned that making the qualification optional is a backward step."