An influential Tory MP has launched a stinging attack on Michael Gove as he called on him to slow down the pace of his school reforms.
Graham Stuart, chairman of the Commons Education Select Committee, said there was a "lack of coherent thinking" in the Department for Education.
Responding to a question at the annual conference of the Headmasters' & Headmistresses' Conference in Belfast on Tuesday, Mr Stuart called for headteachers to speak up and tell the minister to "stop taking the urgency pills".
Stuart was asked what could be done to slow down the rate of change to the exams system.
He told delegates: "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."
Stuart said: "We've got a national curriculum review, they've barely announced that when they announced the English Baccalaureate out of clear blue sky.
"One minute they've said they're raising the benchmark on GCSEs, the next they're scrapping it.
"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."
Ministers announced the first recommendations from their curriculum review in the summer, while plans to replace GCSEs in the core subjects with a new exam called the English Baccalaureate Certificate were revealed last month.
"All we can do is try, it is very difficult, one of my challenges is trying to get them to slow down," Stuart said.
He told the conference that, ironically, the most successful introduction of an exam in recent years was the GCSE, which was implemented over a long period of time.
"'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."
Mr Stuart also condemned the last Labour government's introduction of the Diploma qualification, which combined work experience with academic learning and had been hailed as a potential replacement for GCSEs and A-levels.
"I watched that car crash happen, I was on the committee," he said.
"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.
"But if we got it right, we listened, and got it right, there was a real opportunity."
Then-schools secretary Ed Balls "would not listen" and drove it through, Mr Stuart said.
He added that exam boards said it was the most complicated qualification they had seen and it ended up with "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".