A £50m Government scheme aimed at helping the poorest students go to university is too complicated, a report warned on Thursday.
A new study by the Institute for Fiscal Studies suggests that the flagship National Scholarship Programme (NSP) is unlikely to encourage youngsters from disadvantaged backgrounds to go to university.
The NSP was introduced following the move to triple tuition fees to up to £9,000 a year in a bid to provide financial help to the poorest teenagers.
In its first year the scheme, which replaces an old bursary system, will cost the Government £50m, with the funding matched by universities.
But the IFS study, based on the fees charged and financial support available at 90 English universities, has concluded that the scheme is likely to be undermined by levels of "complexity and uncertainty".
It says that the programme is being administered separately, and in different ways, by each university and that there is "substantial variation" in the type and amount of financial support on offer.
"Universities are free to design their own student support packages, with noticeable differences in the scale and complexity across institutions," it says.
"Some schemes are based on parental income, while others take into account neighbourhood disadvantage; others focus on academic ability or are based on a range of characteristics."
At most universities, would-be students would not know in advance how much total support they would receive, it adds.
Liam Burns, president of the National Union of Students, said there was a need to see an urgent change in the system.
"This report is confirmation that the NSP is currently an overly-complex postcode lottery which fails to provide financial support that reflect the needs of students.
"We need to see real and urgent change to ensure financial support entitlements are known before the student applies to university and then actually follows them and their needs.
"These changes would ensure that those universities with who are successful at widening access do not suffer a financial penalty for recruiting high numbers of poorer but able students, as is currently the case."
The study concludes: "In its current form, the National Scholarship Programme thus seems unlikely to encourage participation amongst students from the poorest backgrounds."
High-ranked universities tend to offer more generous packages, particularly for poorer students, the IFS says.
A student at a leading Russell Group institution with a family income of up to £25,000 could get over £2,900 a year, while those at a newer university, such as those in the University Alliance or Million+ group could get £900 or £700 respectively.
IFS senior research economist Haroon Chowdry, said: "The introduction of the National Scholarship Programme has led to substantial variation across universities in the generosity and type of financial support available to the poorest students.
"Of particular concern is the fact that it is often very difficult for a student to work out how much total support they might receive before they apply.
"This complexity and lack of transparency raises questions about whether the programme will encourage participation among students from poor families."
The IFS also notes that the gap between the numbers of rich and poor 18 and 19-year-old state school students going to university is narrowing.
In 2004-05 there was a 40 percentage point gap and by 2009/10, after fees were raised for the first time to £3,000, this had fallen to 37 percentage points.
This may be because the fee regime, introduced in 2006, was "actually more generous to students from poorer backgrounds and hit richer students relatively harder," the IFS said.
Pam Tatlow, chief executive of million+, said: "We warned that the National Scholarship Programme would be unfair because universities which are most successful in creating new opportunities for students have to bolster the NSP with their own funds. As a result there is a postcode lottery for students."
A University Alliance spokesman said: "These figures are not a useful reflection of the support received by students attending Alliance institutions nor the bigger contribution they make to widening access and social mobility.
"The fact is, Alliance universities have, on average, over twice as many students from lower-income and under-represented groups compared with the rest of the sector whilst achieving some of the highest graduate prospects. That is the bigger picture that these figures fail to recognise."
A spokeswoman for the Department for Business, Innovation and Skills said: "The NSP will double in value next year and help even more students from disadvantaged backgrounds.
"We are always open to hearing how this programme can be improved to make sure that it is targeted at those who need it most, which is exactly why we have reconvened the expert group to make sure we are doing just that.
"Getting to university depends on ability, not ability to pay - and our reforms have ensured that this is now the case.
Applications from students from poorer backgrounds held up last year and the National Scholarship Programme is there to support thousands of those students during their time in higher education."