By Angela Harrison
BBC News, at the ASCL conference
The biggest reform of England's exam system for a generation "could go horribly wrong", Education Secretary Alan Johnson has warned.
Mr Johnson was at the heads' annual conference in London
He told the Association of School and College Leaders conference there was a risk that new practical and academic Diplomas could be seen as second-best.
He suggested this was because they would run alongside GCSEs and A-levels.
Commons education committee chair Barry Sheerman said he should "get off his backside" and do something about it.
Mr Johnson's remarks came in a question-and-answer session after his speech to the ASCL conference in London.
Head teachers put to him their concerns about the planned Diplomas, warning there was a danger they would be seen as lower status than GCSEs or A-levels.
They also said schools and colleges might have difficulties moving pupils around from site to site, as the Diplomas would need to be run by consortiums of schools.
Mr Johnson replied: "Things could go horribly wrong, particularly as we are keeping A-levels and GCSEs.
"The decision was taken in the interests of diversity, so young people have choice.
"That does mean there is a danger of the Diplomas becoming if you like the secondary modern compared to the grammar."
The chair of the Commons education select committee, Labour MP Barry Sheerman, called the remarks "extraordinary" and said the government should "get their act together".
"I don't think it is for the secretary of state to spread alarm and despondency.
"I would prefer the secretary of state to lead from the front and accentuate the positives - get off his backside and do something about it."
The general secretary of the ASCL, John Dunford, called Mr Johnson's comments "refreshingly honest."
He said for the Diplomas to be successful, they must eventually incorporate GCSEs and A-levels.
Dr Dunford said: "With the educational history of this country being littered with examples of first and second class qualifications, the only true guarantee of success will be when the Diploma, embracing both the academic and the vocational, is the only game in town."
The man whose government-commissioned review advocated replacing existing qualifications with a diploma system, Sir Mike Tomlinson, told BBC News the review of GCSEs and A-levels, which the government has promised for 2008, should be brought forward.
The plan is to bring in five Diplomas in pilot areas from next year, increasing to 14 by 2013, when they should be available to all.
They are being designed to be related to various areas of work such as IT, construction and media and will involve work placements.
Head teachers in a consortium of schools in Cumbria hope to be part of the pilot scheme.
Schools and colleges there are collaborating already, so pupils might do some of their lessons at different sites.
Stephen Wilkinson of the Queen Katherine School in Cumbria said the credibility of the Diplomas would rest on their quality and on the backing of employers and university admissions tutors.
Mr Johnson's department later said he had been right to point out the scale of the challenge.
"It would be daft not to recognise that things could go wrong along the way.
"We have to make sure employers recognise the Diplomas, that teenagers are attracted to them and that the education system delivers them properly," a spokesman said.
"That is why we are supporting schools and colleges who will be trailblazing the Diplomas with £50m of support to ensure that they can deliver them effectively."
Conservative schools spokesman Nick Gibb the government needed to ensure rigour and expertise in the delivery of the Diplomas.
"Getting rid of A-levels and GCSEs would not assist in delivering that rigour."