A surprise league-table measure will be introduced every year by the Conservatives if they win the next general election, to ensure schools offer a broad curriculum.

The party, which has been outperforming Labour in opinion polls, said it would pick a different measure each time to stop schools inflating their results by choosing “soft” subjects.

Nick Gibb, shadow schools minister, said too many schools had become obsessed with achieving the five good GCSEs benchmark.

To tackle the problem, a Conservative government would publish an extra column of information, which would not be revealed to schools before pupils sat the exams.

One year it could be the number of pupils achieving seven good GCSEs, including physics. The next it could be the percentage of children getting six C grades, including history.

“We need to get away from the target culture,” Mr Gibb told the Forum of Independent Day Schools. “The target for schools should be to provide a high-quality and broad curriculum.”

Under the Conservative plan, the different results would be chosen from “core” subjects, including geography, history, modern foreign languages and the three sciences. The established measure of five GCSE results including English and maths would remain as a way of gauging improvements.

Mr Gibb said a number of schools had fallen dramatically in the league tables when English and maths results were introduced for the first time in 2006. “It is those kinds of schools that I would be worried about,” he said. “We don’t know what they are sacrificing to focus on a small number of subjects.

“It might produce some quirky results, but it would be a way to encourage schools not to be obsessed with the five or more figure.”

Two top independent schools - St Paul’s in west London and Eton College in Berkshire - have said they will boycott league tables this year because of their negative impact on education.

Last week, The TES revealed allegations that some private schools were manipulating league tables by entering only pupils for exams if they were confident of them securing top grades.

The Specialist Schools and Academies Trust will today publish figures showing that the brightest fifth of pupils are failing to make good progress in state schools where there are only small numbers of more able children. In secondaries with only 20 to 25 more able pupils, just a quarter of them went on to get five A-star or A-grade GCSEs last year. In schools, including selective grammars, with more than 100 bright pupils, more than half achieved five top grades.