Attempts to encourage schools to take up new qualifications by artificially boosting their league-table values should now be thwarted by the new exam regulator, under a change to the law passed last week.
Ofqual has been given a legal duty to oversee the values awarded to different qualifications following claims that ministers' decisions have been distorting the system.
It comes after years of complaints that schools are entering pupils for vocational qualifications of "doubtful value" because they are worth multiple GCSEs in league tables, despite taking a fraction of the time to teach.
Lord Lucas, a Tory peer, told the House of Lords: "Governments tend to introduce new qualifications and want them to be taken up. And one of the ways in which they have incentivised schools to take them up is by seeing to it that they are awarded a substantially excessive number of points ... This distorts the whole pattern of education."
Until now, the Government has decided league-table values after taking advice from the Qualifications and Curriculum Authority.
Lord Lucas wanted the power taken away from ministers and given to exam regulator Ofqual. Instead, the Government, hurrying to pass its Apprenticeships, Skills, Children and Learning Bill, offered an amendment saying Ofqual "must keep under review any system used by the Secretary of State for allocating values to qualifications".
Baroness Morgan, children's minister, agreed that any suspicion of ministerial interference risked "undermining the credibility of the school accountability system".
"The values put on qualifications in performance measures for schools should reflect both their challenge and their size, and how much time it typically takes to study for them," she said.
But academics claim the Government has been failing to stick to that principle when deciding the values of its new Diplomas.
If pupils pass a higher Diploma, including functional skills tests, their schools gain the equivalent of eight-and-a-half A*-C GCSEs in the league tables, even though they have been advised to allocate just two days a week to teaching the qualification.
The Government originally suggested the qualification would be worth five or six GCSEs.
Professor Alan Smithers from Buckingham University has called the tariff "ridiculous". "This looks like a blatant attempt by the Government to incentivise schools to encourage their pupils to take this qualification," he said.
Ofsted has said that vocational ICT qualifications - worth four A*-C GCSEs in the league tables, but only needing the teaching time of one - were of "doubtful value".
Lord Lucas, who backs the new law, told the Lords: "Schools plunge into these exams not so much because they are what are required by their pupils, but because it is the way in which the schools can be rated highly in the league tables. That is not the motivation which should lie behind a school's decision."
The Tories are opting for a very different solution, with a pledge to remove vocational qualifications from academic league tables if they win power next year. Lord Lucas said he disagreed with the policy as it could discourage pupils from taking a mix of academic and vocational subjects.