I’ve asked universities minister Jo Johnson to come clean. He refuses. So it’s time to reveal the hidden parental contribution that a Conservative government expects families to pay for their children’s university education.

They’re 18 — that’s old enough to vote, marry or join the army. True independence! Well, not quite if they’re off to university. All first-time UK undergraduates in England are eligible for a full tuition fee loan. Yet for most under-25s the amount of their maintenance loan (for covering their living costs) is increasingly dictated by an assessment of their parents’ income.

That means most do not get the full loan, as even £25,000 family residual income (roughly, income minus pension contributions) is enough to start reducing it. And while back in 2015 all students received at least 65 per cent of the maximum, now some with a parental income of more than £58,000 get less than half.

Student debt is an election issue. As well as scrapping tuition fees, Labour proposes reintroducing grants — for now I’ll leave the debate over who pays, the taxpayer or the individual, to politicians. Yet while studying, what most students need, whether via maintenance loan or (likely means-tested) grant, is enough cash to live off.

The implicit premise is that parents will fill the gap — but implicit isn’t good enough. It should be explicit. Yet this fact, never mind the actual amount, is hardly touched on in any official literature.

All I can find is one flaccid mention buried in the Student Loan Company’s How You’re Assessed guide, which says: “Depending on their income, parents may have to contribute towards your living costs while you’re studying.”

That isn’t good enough. Parental income is the only element that dictates how much of the full maintenance loan a student gets. To be transparent and fair to both students and parents, the loan letter should say something like this (based on a student starting university in September 2017, living in London, with £70,000 combined family income): “Students — your maintenance loan is £5,479 a year. This is less than the full loan and we expect your parents to make up at least the £5,523 difference.”

That doesn’t happen. The offer letter only says the loan amount. It doesn’t say what the maximum loan amount available is. At my TV roadshows I often hear parents complain, “it’s a disgrace the maintenance loan isn’t enough to cover their rent — I have to scrape up extra”. However, when I question them, these are usually parents whose offspring don’t get the full loan. It isn’t an accident — the “extra” is that hidden parental contribution.

This is not a trivial issue. It is often said “you don’t need to pay upfront to go to university”, but the sharp rise in the amount of means-tested maintenance loans means that is no longer true for many. Why aren’t parents being told that (rightly or wrongly) the system design incorporates a contribution from them?

This lack of transparency and clear guidance causes friction between students and parents. It also leaves some students in a dire position.

While parents are supposed to contribute, many can’t or don’t, or see the amount as loose. This leaves some students risking unmanageable debts or dropping out of university over cash flow issues.

How much are parents expected to contribute?

As the government won’t tell you explicitly, let me at least set it out here. This brief table shows one year’s full maintenance loan amount for students who will start this September.

The student loans gap Student living at home Student living away from home Student living away from home (London) Full annual maintenance loan £7,097 £8,430 £11,002 Source: Student Loans Company

If your loan offer letter is less, subtract that from the maximum loan to see your official parental contribution. For those already at university the numbers are different, so I’ve full ready reckoners at mse.me/parentalcontribution.

This lack of transparency isn’t new. On the back of the large increase in the proportion of expected parental contribution, I wrote to the universities minister Mr Johnson a few months ago, suggesting it was time to clear it up.

His flaccid and confusing response argued that just because the calculation is based on parental income it doesn’t mean parents are expected to make a particular contribution — students can make up the difference from savings or part-time jobs.

Of course that’s true, but that applies to all students regardless of parental income — so why differentiate based on parental income at all? On that logic, all students should get the same maintenance loan. If we don’t expect parents to make up the gap, why judge on parental income?

If my suggestion for the offer letter is too didactic, it could at least be changed to say:

“Students — your maintenance loan is £5,479 a year, which is £5,523 less than the full loan as your parents have a higher income.”

Bizarrely, while student maintenance loan amounts depend on parental income, there’s no obligation on parents to contribute (and students can’t force them to).

The finances of students and their parents should either be considered separately, or students should have some ability to be able to force parents to comply.

While many headlines focus on demonising the “huge debts”, the biggest practical problem some students face is that the maintenance loan isn’t big enough

Some under-25s can have their finances declared independent from their parents and thus separately assessed, but the criteria is extremely tough. Students will need to prove that they have supported themselves financially for three years before starting university.

The irony of the student finance debate is that while many headlines focus on demonising the “huge debts”, the biggest practical problem some students face is that the maintenance loan isn’t big enough. With rents rising, even the full loan amount can leave some struggling to make ends meet.

Of course, bigger loans are a psychological deterrent to many from non-traditional university backgrounds — which is why proper financial education is needed to explain that the unique way student finance works means often it won’t cost them more.

Student loan repayments are set at 9 per cent of everything earned above £21,000 for 30 years, regardless of how much you borrow. So unless you are a big enough earner to clear the borrowing and interest within the 30 years bigger “debts” don’t actually result in you repaying more (see my FT article The government sold millions a lie on student loans).

Martin Lewis is the founder of Moneysavingexpert.com and is former head of the Independent Taskforce on Student Finance Information