Thousands of pupils in the UK are being given scant or wrong advice about the best A-level subjects to study to gain a degree place, a survey has found.
The study by The Student Room online forum suggests many students have poor guidance on what to take at A level.
Of more than 6,000 students in the study, hundreds said they found they had taken the wrong subjects to access a chosen university course or career.
The Student Room called it a "black hole" in school careers advice.
Almost a third (32%) of those who took part in the study rated their school's careers advice as "weak".
About a quarter (23%) said they did not have enough information to make informed choices about their future careers or the subjects they should study to achieve their ambitions.
The Student Room says the students who felt they received the worst guidance were often from the poorest backgrounds with no history of higher education in their family.
Among less affluent students, 39% said they had received poor information on subject choices.
Hundreds of students found their ambitions thwarted because they had not realised maths A-level was essential for a raft of degrees, including most sciences, engineering and computing, and is also sometimes required for philosophy courses, says the report.
"My school didn't tell me that maths was a requirement for the majority of chemistry and natural sciences courses, which means I am now very limited," one student commented.
Another said: "I was told further maths was only needed if you wanted to do maths at uni and now I'm at a disadvantage when applying for engineering."
Yet another lost out on a place to study medicine at one university after being wrongly told A-level biology was not needed for this particular course.
The confusion is not just limited to science and maths subjects.
"We were made to do general studies... which most universities don't actually count," said one student.
Another complained: "I had absolutely no clue that history was an extremely desirable subject when applying for an English literature degree."
The Russell Group of universities, where competition for places is intense, has published guidance advising students to pick at least two A-levels from a list of "facilitating subjects" to maximise their chances.
But the survey suggests this message is not getting through to schools.
One student said: "You're told to pick subjects which you enjoy and are good at. So I took a total mismatch of subjects with no real end goal and nobody said to me that I might struggle to find a university course because of my mixed set of A-levels."
"There is a black hole in school careers advice," said Student Room chief executive Jason Geall.
He called for the winners of the next general election "to make immediate and long-lasting changes to schools' careers advice, so that students are properly informed, can fulfil their lifetime ambitions and do not have any regrets".
A Department for Education spokeswoman said the government was "taking decisive steps to improve careers advice".
She said: "All schools have a statutory duty to provide independent careers advice and are held to account by Ofsted on the standard of the advice they offer.
"Our updated guidance, published in April, makes clear that schools should involve employers in careers advice and ensure pupils are informed about all options, from A-levels and university to vocational routes such as apprenticeships."