Baroness Verma of Leicester, shadow minister for children, schools and families and universities and skills, told a fringe meeting on Tuesday that Diplomas were a "back door" qualification.
"Diplomas were brought in for children who were not so good at GCSEs," she said. "They cannot be seen to be the panacea for all of the ills that GCSEs and A-levels do not address. At the moment, they are this back-door route into getting a qualification."
She added: "We should not rush into things like this - they need to be properly tested so we know what the full impact will be."
But a former head at the meeting, who did not want to be named, said the peer had got it "totally wrong".
She said: "Baroness Verma clearly has a very vague notion of what a Diploma is. She seemed to be maintaining that it was a low-level qualification. She obviously hasn't been briefed adequately. She then tried to change her argument and say the programme had been rushed, but that's also not true."
The Diploma was meant to end the academic-vocational divide, with ministers hoping they would become the qualification "of choice".
Fourteen new work-led Diplomas were announced by the Government in 2005. Three academic Diplomas - in humanities and social sciences, languages and science - were later announced. They will be offered in 2011.
The Tories have shown little inclination to embrace Diplomas if the party gains office, publicly stating they need a rethink while privately suggesting they may be scrapped.
Michael Gove, shadow schools secretary, has already promised to remove vocational courses from league tables. He believes pupils are pushed into "soft subjects" by teachers to improve schools' ratings, skewing the data used in performance tables.
Speaking at the Labour conference last week, Schools Secretary Ed Balls said Mr Gove will simply exacerbate the two-tier vocational-academic system.
Plans in the making
- Build 12 new technical colleges
- Turn schools in special measures into academies
- Give all schools opportunity to become academies
- `Outstanding' schools to be pre-approved to become academies
- Ofsted's school inspection reduced from 18 categories to four
- `Outstanding' schools to be exempt from Ofsted inspection.