Half of Scottish universities are failing to meet the international average of student-to-staff ratios, raising fears about the quality of education on offer.

Official figures show that, on average, higher education institutions in Scotland have a member of academic staff for every 15.5 students.

Although this is the same as the OECD average, lecturers argue that in order to compete with the best institutions in the world, Scotland should make a priority of cutting student-to-staff ratios.

According to the Higher Education Statistics Agency (HESA), the highest number of students per staff in Scotland is at Queen Margaret University College in Edinburgh, which has a ratio of 20.3.

The universities of Robert Gordon in Aberdeen, Paisley and Glasgow Caledonian all have ratios of more than19 followed by Abertay, in Dundee, and Napier, in Edinburgh, which have ratios of over 18. Strathclyde University and Heriot Watt both have a ratio of 17.3.

The call for more staff from the Universities and Colleges Unions in Scotland (UCU), which represents the majority of lecturers, comes at a difficult time for universities.

Although student numbers have remained static since 2000, the previous decade saw an unprecedented expansion of higher and further education which pushed participation rates up to 50% of school-leavers. However, in recent months some Scottish universities have put in place redundancy schemes following a pay rise for lecturers last year and other cost pressures such as pensions and rising fuel bills.

Tony Axon, research officer for UCU Scotland, said the expansion of student numbers and the decline in staff was damaging the quality of education on offer.

And he warned that because lecturers spent much of their time doing research and preparing for lessons actual class sizes were much higher than the suggested ratios.

"Ratios paint a false picture because staff are not simply teaching. The real situation is that students are being taught in classes of up to 200 which we believe is completely unacceptable," he said.

"If we want to compete with other OECD countries we should have an ambition to bring down ratios sharply in line with the best of our competitors, but that is not happening. We need more money for staff."

A spokesman for Universities Scotland, which represents university principals, said "There are cost pressures and universities are having to manage these, but the quality of what is on offer in our universities is internationally recognised.

"Overseas students who have an experience of higher education in different countries rate Scotland very highly in terms of the quality."

Universities Scotland also highlighted Scotland's superior position to England, where - at 16.8 - average student staff ratios are higher than the international average.

Sally Hunt, UCU joint general secretary, said: "Despite a recent recalculation of figures the UK is still way behind competitor countries, particularly Germany and Japan, and the ratio of students and staff actually rose this year."