An influential Tory MP and England's exam regulator have raised concerns about the pace of proposed changes in the exam system.
Graham Stuart, the Education Select Committee chairman, told a conference of private school heads that GCSE reforms lacked "coherent thinking".
Ofqual chief Glenys Stacey told heads she would intervene if the timetable for change was unachievable.
The Department for Education said the exam system has "deep problems".
Speaking to the Headmasters' and Headmistresses' Conference (HMC), Mr Stuart urged heads to tell Education Secretary Michael Gove to "stop taking the urgency pills".
He was addressing concerns about changes to exams in England, which from 2015 will see GCSEs in some core subjects replaced by a new qualification called the English Baccalaureate Certificate.
This is in addition to an overhaul of GCSEs which will see pupils taking final exams, rather than receiving marks for individual units.
"One minute they've said they're raising the benchmark on GCSEs, the next they're scrapping it", Mr Stuart said.
"We've got a national curriculum review, they've barely announced that when they announced the English Baccalaureate out of a clear blue sky.
Responding to a question on what could be done to slow down the rate of change to the exams system, Mr Stuart admitted "one of my challenges is trying to get them to slow down".
He told the annual meeting of leading private school heads: "Please speak up loud and clear. You may be dismissed initially, the more voices who say to the secretary of state 'stop taking the urgency pills' and recognise the need to slow down."
"Now if there's coherent thinking going on in the department, then so far it's passed by the chairman of the Education Select Committee."
"'Speak up' is all I can say. At the moment they don't appear to listen and it doesn't look very coherent. When you throw in on top of that incoherence urgent time lines which can't be done, you're looking at a mess. My key message at the moment is stop changing everything all the time."
Head teachers at the conference also discussed the exam changes with Ofqual chief, Glenys Stacey.
She told the conference she would tell the government if the timetable for implementing the new exams was unachievable or "if the risks to standards or delivery are unacceptable".
She said Ofqual had not yet reached a view on whether the timetable for the changes was possible - "but there is frankly a lot to understand and work to be done in the coming months to evaluate that".
A spokesman for the Department for Education said: "We are trying to move quickly because the current system has deep problems, as everyone now sees, and it is entirely reasonable for the head of Ofqual to be cautious and careful, given all the problems with the exam system."
Mr Stuart also said the Labour government's introduction of the Diploma qualification had been "a car crash".
The Diploma combined vocational with academic learning and was claimed as a potential replacement for GCSEs and A-levels.
He said that the then Education Secretary, Ed Balls, "would not listen" to concerns and drove it through.
"Here was an opportunity, there was the money, there was the political will, there was a real opportunity to create a new vocational qualification, over time. It takes 10 years, we were told by the awarding bodies, to establish a new qualification.
The result was "tens of millions of pounds wasted on it, children put on it ended up in a cul-de-sac and abandoned, and I hope and pray this government isn't going to do the same thing", said Mr Stuart.