Diplomas involve a mix of practical and academic subjects
England's new secondary school qualification, the Diploma, is not stretching the brightest pupils, the exams watchdog says.
Ofqual is warning that standards of attainment shown by Diploma pupils were lower than in those taking other exams.
The Diplomas are intended to mix vocational with academic learning. Ministers have said they could eventually replace A-levels.
Ofqual says the qualifications are good but "need finessing".
In the first report into the content of the qualifications, Ofqual looked at engineering, which is offered by AQA-City and Guilds; society health and development, offered by Edexcel; and creative and media, which is offered by OCR.
The watchdog concluded the qualifications - all level 2 diplomas aimed at 16-year-olds - were not sufficiently challenging for the targeted students.
"There was evidence across the Edexcel and the OCR specifications that candidates' responses had been limited by poorly designed consortia/centre-set tasks," the report said.
Head of Ofqual Kathleen Tattersall said: "These reports are an important part of our work to safeguard standards in these new qualifications.
"We shall be carefully reviewing all the findings, including those of the principal learning qualification where it was found that some question papers did not provide sufficient opportunities for the more able candidates to demonstrate the extent of their knowledge, understanding and skills.
"We will continue our monitoring of these new qualifications and will continue to work with awarding organisations and other stakeholders to ensure that they are fit for purpose and valued by learners, teachers, parents and employers."
Ofqual has advised the awarding organisations to agree action plans for future examinations.
The bodies behind the Diplomas examined by Ofqual say the qualifications are being successfully taught in schools.
A statement from the three Diploma Development Partnerships mentioned in the report said: "We were gratified by the report, which essentially showed that the qualifications were of high quality, but needed some finessing in terms of assessment tasks.
"This conclusion echoes the views expressed in Ofsted's 2009 early review of Diploma delivery.
"Overwhelmingly, Diplomas are being successfully taught in our schools and colleges and young people are responding well to the innovative and exciting learning environment. Employers are viewing Diplomas very positively, and the learning methods prepare students extremely well for work and/or higher education."
The take up rate of the qualifications has been lower than originally intended.
Figures released in November showed 11,326 youngsters took the qualifications in their first year of operation.
Pupils can opt to take a course at three different levels: Foundation, which is equivalent to five GCSEs at grades D-G; higher, which is equivalent to seven GCSEs at grades A*-C; and advanced, which is equivalent to 3.5 A-levels.
The first five courses were introduced in some schools in September 2008.
These included engineering, construction, IT, creative and media, and society health and development.
A further five - in business, environmental studies, hair and beauty, hospitality and manufacturing - were introduced 12 months later.
The Conservatives have said they would scrap three new academic Diplomas - in science, modern languages and the humanities - which are due to be introduced in two years' time.