Universities are warning the Erasmus study abroad scheme will have no more funding for UK students in the event of a no-deal Brexit.
It would mean that 17,000 UK students would not study in European universities as planned next year.
The UK government had previously indicated it wanted to continue taking part in the exchange scheme.
But Universities UK says new advice last week suggested no further support would be available in a no-deal Brexit.
The universities organisation says, as a "matter of urgency", the UK government must "reconsider its decision" so that financial backing will be assured for students in the exchange scheme.
Universities UK is launching a campaign calling on the government to commit to funding study-abroad placements for 2019-20 and beyond, even if there is no deal on Brexit.
They say the updated information means that students already studying abroad will continue to be supported, under the current Erasmus+ scheme, but there is no provision beyond that.
Alistair Jarvis, chief executive of Universities UK, says studying abroad has many benefits, including making students more employable and improving their language and communication skills.
"An investment in international experience for our students now is an investment in the future of our economy," he said.
The Erasmus exchange scheme for students in Europe has been operating since the 1980s - and gives students the chance to study in another country for three to 12 months as part of their degree.
Even after Brexit, the UK government had said it would continue to take part, such as paying to join as an associate member, along with other non-EU countries.
The government's published plans on the UK's future relationship with the EU committed to "continue to give young people and students the chance to benefit from each other's world-leading universities, including cultural exchanges such as Erasmus+".
But Universities UK is warning that seems to have been cast into doubt.
The latest government position on Erasmus is that it "aims to minimise the impact on projects in a no-deal scenario" - but future participation will depend on negotiations.
"The government will need to reach agreement with the EU for UK organisations to continue participating in Erasmus+," says the updated official guidance.
The government's advice says it is "seeking to hold these discussions with the EU".