State-educated students may get less help with university applications, struggle to find experiences to write about and are more likely to make mistakes, research suggests.
A study commissioned by the Sutton Trust claims that teenagers educated in the state sector are at a disadvantage when writing their personal statements to send to universities.
It suggests that the current system favours private school pupils, who receive help to submit "carefully crafted" applications that contain details of numerous top work placements and after-school activities.
Dr Steven Jones of Manchester University looked at the personal statements of just over 300 would-be students, all with the same grades, from a mixture of private, comprehensive and grammar schools, and sixth form colleges.
Students applying to a UK university complete a personal statement giving details of their work experience, extra-curricular activities and any other information they believe would be relevant to their application.
Ucas has said that the statement was a way for a potential student to "stand out from the crowd".
But Dr Jones' study claims that the statement in its current form may be unfair to those from lower or middle income homes.
The findings suggest that private school students are more likely to list a higher number of work-related experiences, and these could include stints working in places such as banks, law firms or leading businesses.
State school pupils listed a slightly fewer number of work placements, and these were more likely to include Saturday jobs and visits to businesses arranged by the school.
The report also said that writing errors were three times more common in personal statements written by applicants at sixth form colleges than those from independent schools.
It concluded that the personal statement was often said to make the university applications process fairer.
"This research challenges that assumption, finding that independent school applicants are more likely to submit statements that are carefully crafted, written in an academically appropriate way, and filled with high status, relevant activities," it said.
"By contrast, state school applicants appear to receive less help composing their statement, often struggling to draw on suitable work and life experience."
Sutton Trust chairman Sir Peter Lampl said: "This research suggests that the personal statement further disadvantages from low and middle income backgrounds.
"Good state schools and colleges already help their most able students apply for places in leading universities.
"This should become the norm, and groups of state schools and colleges should do more to arrange support for the admissions process locally.
"But admissions processes also need to change.
"Personal statements should be more than an excuse to highlight past advantages.
"Applicants should outline how they might contribute to campus life, and universities should make it clear that applicants are not penalised for lacking opportunities in the past due to family circumstances."
The Trust said that personal statements could be made fairer by limiting the number of experiences an applicant could include; schools and colleges offering students more support, and judging applicants from low and middle income homes based on the academic and extra-curricular activities available to them.
Ucas chief executive Mary Curnock Cook, said: "The Sutton Trust is right to highlight the importance of support and advice about how to construct a personal statement which will enhance an application to higher education.
"However, I have every confidence in the professionalism of admissions officers - they are highly experienced in recognising social factors in the content of personal statements.
"As we say in The Ucas Guide to getting into University and College, demonstrating a strong rationale and passion for the course chosen is the number one requirement in a strong personal statement.
"The personal statement is one of a number of pieces of evidence that admissions staff look at when assessing an applicant's potential to succeed in higher education."
Nicola Dandridge, chief executive of vice-chancellors' group Universities UK, said: "The report does raise the important issue of how school type, background and access to professional networks can influence the experiences of young people.
"However, it is important to remember that university admissions staff are highly experienced at recognising this and taking such things into account when interpreting personal statements. It is also important to be aware that the personal statement is only one factor universities take into account when selecting applicants.
"The research also raises the importance of young people having access to high quality advice and guidance when preparing a personal statement."
Dr Wendy Piatt, director-general of the Russell Group, which represents 24 leading universities, including Oxford and Cambridge, said: "As our universities make very clear on their websites and elsewhere, personal statements are primarily an opportunity for applicants to show their academic interests and reasons for wanting to study a particular subject. Work experience and extra-curricular activities should be a relatively small component of this and are rarely a major factor in admissions decisions.
"We agree with the Sutton Trust that it's really important that students receive high-quality information, advice and guidance, including on writing personal statements.
"For that reason Russell Group universities provide outreach activities and give lots of help and advice to teachers, especially those working in deprived areas, and have also organised free conferences for teachers.
"Our admissions tutors are expert professionals who take into account a wide range of factors to give applicants every opportunity to demonstrate their strengths and interests in the subject."