England's exams regulator has indicated that she may have concerns about the Government's proposed timetable for introducing its new English Baccalaureate.
Glenys Stacey said that Ofqual needed to understand more about the detail of the planned curriculum so that it could shape the qualifications.
But she warned the regulator would not be afraid to speak up if the timetable for introducing the new exams was not achievable.
She suggested that there was still a lot to understand about the qualifications and work to be done on it.
Addressing private school heads at the HMC conference in Belfast, Ms Stacey said: "It's early days, we at Ofqual need to understand more about the detail and about the curriculum so that we can properly envisage and then shape the qualifications.
"Some things are worth saying straight away. Firstly, we will advise government on the timetable for change and we will say if it is not achievable, or if the risks to standards or delivery are unacceptable."
She later added: "We will always advise government on the timetable and will say if it is not achievable or if the risks to standards are unacceptable. We will say that and ministers will listen to that. Is the timetable achievable?
"Well, we are not saying at the moment that we have come to a view that it is not possible to achieve it, but there is frankly a lot to understand and work to be done in the coming months to evaluate that."
Ms Stacey added that headteachers in the auditorium should note that when they were asked for a show of hands if they were concerned about the timetable "I put mine up".
Ministers' plans to replace GCSEs in the core subjects with a new exam called the English Baccalaureate Certificate were revealed last month. The courses would be first taught from September 2015, with the first exams taken in 2017.