Figures collated by The Herald show students from poorer areas in Scottish higher education have fallen from 28% of the total in 2002 to 27.2% in 2011.
The fall is significant because it has come during a time when successive Scottish governments have pledged to prioritise fairer access to university.
Despite this, the universities of St Andrews, Edinburgh, Glasgow, Aberdeen and Dundee have all seen the proportion of such students drop.
And only four universities have seen proportions increase – Strathclyde, in Glasgow, Heriot-Watt and Queen Margaret in Edinburgh, and Stirling.
It has led to calls from lecturers and students for renewed efforts to find a solution that improves the balance.
The situation has arisen because access to university is tightly controlled, with pupils given priority depending on their exam results.
Because poverty has a direct impact on academic achievement, pupils who are bright enough to go to university can fail to get the qualifications that allow them to do so.
To counter this, universities have developed schemes to attract those from poorer backgrounds – with some institutions offering lower entry grades to those who complete access courses.
Universities have also been asked to accept students into the second or third year of degree courses once they complete Higher National qualifications at colleges – so-called articulation.
More recently, the Scottish Government announced plans to give universities binding targets on widening participation, with the prospect of fines for those that fail.
However, universities opposed the move, arguing the problem is largely not of their own making because of the markedly differing attainment between pupils from deprived and middle-class areas that can develop in primary school.
Gordon Watson, president of UCU Scotland, which represents lecturers, believes it is time for drastic measures to redress the balance. He supports the idea of universities taking top-performing pupils from state schools in deprived areas, regardless of their exam results.
"The fact remains that huge swathes of these young people are simply not considering university," he said.
"We need to look at more effective ways to get the lowest performing schools to send more students to our universities."
Robin Parker, president of NUS Scotland, said improving access to university was crucial in developing a fairer society. He added: "Universities aren't doing their jobs properly if they overlook talented people from poorer backgrounds."
Alastair Sim, director of Universities Scotland, said: "The data should not be interpreted as a lack of commitment among universities to widen access to bright learners whatever their social backgrounds.
"To deliver significant change in universities, you first need to tackle the root of the problem which is the large gap in attainment according to deprivation in schools as recent reports have confirmed.
"Universities are committed to working with the Scottish Funding Council on access agreements which will be in place for 2012/13. We are working to open up opportunities – such as articulation, contextual admissions, summer schools and close engagement with primary and secondary schools – and will be looking at what more can be done."
A spokesman for the Scottish Government added: "We have seen an increase in participation rates in higher education in recent years, including a narrowing of the gap between the proportion of students from the most and least deprived areas. However we are determined to do better, which is why we will be introducing statutory widening-access agreements."
Some 47% of the population, excluding long-term unemployed, belong to socio economic classes 4 to 7. This includes taxi drivers, hairdressers, plumbers, electricians, mechanics, factory workers, shelf stackers, shopkeepers, labourers and cleaners.
As higher education has expanded, the number of working class students has risen slightly since 2002/03 – from 5175 to 5430 – but the overall proportion has fallen because more middle class youngsters are going to universities.