Plans to allow students in England to study for honours degrees over two years will "devalue" the qualification, the lecturers' union Natfhe claims.
The traditional degree course takes three or four years
The new "fast-track" degrees will be piloted at five universities from September, enabling students to start working sooner - and with less debt.
Higher Education Minister Bill Rammell said flexibility was needed to open up higher education to more people.
But Natfhe fears the move will encourage a culture of "cramming".
The union also warns that university staff will lose out on time for research and updating course material.
The fast-track courses will be piloted at Staffordshire University, Derby University, Leeds Metropolitan University, University of Northampton and The Medway Partnership in Kent.
Roger Kline, head of higher education for the union, said the proposal had serious implications.
"We believe it fails to acknowledge the importance of developing critical and analytical skills while studying for a degree," he said.
"Reducing study time may diminish the degree experience by replacing considered study with intensive 'cramming' - this would not develop the skills we seek from graduates.
The "fast-track" degrees will be piloted at five universities
"We do not believe that, for the overwhelming majority of students, a two year degree will do anything other than devalue the worth of a degree in the eyes of both employers and peers."
Mr Kline said there was a danger of "rushing through" the initiative.
"We think there needs to be a lot more thought and evaluation before we charge off in this direction - because we can see some major problems which need sorting out, " he told the BBC News website.
But Mr Rammell said there was "no question of 'devaluing' degrees".
"Anyone studying a condensed degree would have to have the same knowledge and understanding as those studying for three years - there is no question of lowering the bar," he said.
"While a two-year degree will not be right for many students who want to take longer to study their subject and enjoy university life, it's right that we expect service providers to have scope to offer flexibility, not uniformity."
Impact on staff
Mr Kline also raised concerns about staff having to teach all year round, without adequate time for "research, scholarly activity and subject updating".
"The attractions to the government of two year degrees are obvious - less student debt, cheaper degrees and more chance of hitting their 50% participation target," he said.
"We believe no further pilots should be run until the evaluation of the existing pilots has been published for wider consultation and debate.
"If the government is serious about doing something about the burden of fees and widening participation, it should immediately create a level playing field for part-time, mature students - not rush to introduce another untried initiative."
The traditional full-time course pursued by most undergraduates takes three years - or four in Scotland.
The move to reduce the time it takes - first announced in 2003 - marks a new effort to increase the proportion of young people with higher education qualifications.
The government has a target of 50% by the end of the decade, but projections suggest it might struggle to meet this.
From this autumn, full-time undergraduates face tuition fees that will in most cases be £3,000 a year, compared with the present £1,175.