The caveat was delivered to Michael Wills, the minister for learning and technology, at a recent visit to the society's headquarters.
The BCS says it has seen a 35% increase overall in the number of students to staff over five years on the degree courses the society accredits as counting towards its professional qualifications. At some universities the rise has been more than 70%.
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Last year a record 14,600 students signed up for computer science courses, a 15% increase on 1998.
"We are seeing growing evidence of deteriorating staffing ratios and anecdotal evidence of unfilled vacancies in junior teaching posts and PhD positions," says BCS chief executive Judith Scott in a follow-up letter to Wills.
"At one institution there were more than 10 teaching posts vacant at the start of this year," she says.
"The population of students registering for computing courses continues to rise inexorably.
"Given that we have a skills shortage in IT, we are facing the prospect of producing large numbers of graduates receiving a declining quality of teaching.
"This will lead to more complaints from industry that the universities are not providing the calibre of graduates they need."
Scott says pay is a key issue here.
"The effect of student debt combined with the huge gap between academic and industrial pay is a major influence.
"Why would a young graduate with debts of £5,000 take a position on a PhD grant of £6,500 a year, to be followed by a starting salary as an academic of £16,000 when they can immediately get a job in industry with a starting salary of £18,000-£20,000?
"Departments report that academic salaries are generally 30%-50% below those in the commercial sector. Merely increasing individuals' pay by reducing the number of staff will not work. Teaching ratios are already threadbare in many places. This will just make them worse. Computing needs more money if we are to solve this problem, and have a thriving academic community of researchers and lecturers."
Scott says the ratios of students to staff have worsened more at the new universities, the former polytechnic, probably because they tend to have fewer people working towards PhDs: researchers usually also teach.
She concludes, "I would be grateful if you could draw this issue to the attention of the minister responsible for higher education, and seek some action on what is rapidly becoming a major problem."