By Gary Eason
Education editor, BBC News website
The government and the universities admissions service are using different ways to compare the International Baccalaureate (IB) and A-levels.
A-level results tend to improve each year
The government's school league tables will now count the top IB score as equivalent to five A grade A-levels.
But admissions service Ucas has decided it is equivalent to more than six.
Universities are finding it hard to differentiate between the best A-level candidates, and more schools are offering students the IB.
Government and Ucas rankings are more closely aligned for average candidates.
The middle IB score would equate to about three and a half A grade A-levels under both.
They are adrift at the lower end, with a basic IB pass being worth 2.8 top A-levels in league tables but only 2.3 to Ucas.
The government's new league tables points have been devised by the Qualifications and Curriculum Authority (QCA).
It used to use the Ucas tariff but is switching away from this because it wants to include a wider range of vocational qualifications.
In a 2004 report on the comparability of A-levels and the IB, the QCA said decisions that were made "may affect the whole system".
It added: "Any recommendation about how to translate such comparability into points for use in the performance tables has the potential to make the IB more or less attractive to centres and could therefore be seen as politically motivated."
Some 87 UK schools have adopted the IB.
Candidates take three subjects at higher level and three at lower. They write an extended essay, take a course in the theory of knowledge and complete a component called "creativity, action and service".
A Ucas expert group compared three IB subjects - geography, mathematics and chemistry - with A-level syllabuses offered by the Edexcel, AQA and OCR exam boards.
It concluded that someone getting the maximum possible IB points should have a tariff score of 768.
This is 6.4 times the 120 points given for an A grade at A-level.
The report notes: "A-level does not recognise the difference between a good pass and a bare pass at grade A."
In practice hardly anyone gets the top IB score. The median score has been assigned a tariff level of 419, equivalent to an A and three Bs at A-level.
In the alternative QCA system, a top IB score merits 1,380 points, while an A grade A-level contributes 270.
The median IB score gets 930 points.
A government-commissioned report by Sir Mike Tomlinson recommended a diploma at different levels, which would include more challenge for the brightest.
Ministers rejected this in favour of retaining A-levels. But they have asked the QCA to look into making them more demanding.
The Ucas report says the relationship between the IB and A-levels might change when the new A-levels are introduced in 2008.
The general secretary of the Association of School and College Leaders, John Dunford, said of the Ucas decision: "It reinforces the Tomlinson view that broader qualifications are of great benefit and that the 'three A-level' course remains a very narrow range of study for 16 to 18-year-olds."
The International Baccalaureate Organization was delighted by the Ucas tariff.
Director general Jeffrey Beard said: "As well as having a positive impact on higher education admissions for IB diploma students in British schools, the tariff will also benefit many more students who wish to continue their education in the UK after studying for the diploma overseas."