By Sean Coughlan
BBC News, at the ASCL conference
Head teachers have warned that confusion and a low take-up rate could doom England's new Diploma qualification to second-class status.
John Dunford is passing on school and college leaders' concerns
But Schools Secretary Ed Balls has restated his "optimism" about the success of the qualification.
Heads' leader John Dunford says there must be a less complex Diploma system, incorporating A-levels.
The government has announced a new Extended Diploma, this advanced version being worth four and a half A-levels.
Speaking at the annual conference of the Association of School and College Leaders, Dr Dunford gave ministers a stark warning about the risks of failure of a qualification which is due to be launched this autumn.
Introducing an additional upper tier of the qualification would make the structure of the qualification even more confusing, he warned.
"People are not going to go for a qualification which is too difficult to understand," Dr Dunford said.
He said that heads were supportive of the principle of the Diploma, but unconvinced by the viability of the current proposals.
The complex structure and lack of clarity about how it will operate alongside A-levels meant that pupils and schools might reject it, he said.
And he warned that rather than bridging the divide between academic and vocational qualifications, an unsuccessful Diploma could reinforce the idea of a two-tier system.
But in a question and answer session with head teachers, Mr Balls repeated his belief that the Diploma was on course to succeed.
"I'm optimistic that we can make this work," he told heads. "It's a very big and difficult thing to do," he said – but that there was no need for a "defeatist" approach.
Mr Balls also left open the door to further changes in the structure of the Diploma, including the head teachers' proposal that A-levels and GCSEs should be included in an over-arching Diploma, rather than being in competition with the new qualification.
The schools secretary indicated that this use of a Diploma as a "wrapper" around other qualifications had not appealed to him initially – but that he would not rule it out.
The speech from Mr Balls was well received by head teachers – but on Diplomas he faced a sceptical audience.
The creation of the Extended Diploma would mean that schools and groups of schools would be expected to deliver 119 different permutations of Diploma, said Dr Dunford.
"The problems with adding another layer of complexity are twofold. It becomes more difficult to implement in schools and it makes it much harder to explain to students, which means the take-up will not be as high," said Dr Dunford.
Even on a logistical basis, bussing pupils between different schools for their Diploma options could prove impractical.
A Department for Children, Schools and Families spokesperson said: "We know that schools, universities and employers support Diplomas but naturally there will be discussions about how best to implement them.
"We realise that schools and colleges will need extra money to do this; as a result they will get around 10% extra on top of what they already receive.
"It is absolutely not the case that any one school or college will be expected to deliver 51 Diplomas and we have deliberately staggered the implementation of Diplomas so that schools and colleges will not feel overwhelmed."
Shadow children's secretary Michael Gove said: "It's vital that the Diplomas are high quality.
"It's equally important that A-levels survive. Ed Balls has again ducked the chance to guarantee he will keep this gold standard qualification."
Liberal Democrat spokesman David Laws said: "This further tinkering with the qualifications system seems to be a desperate attempt to get Diplomas back on track."