Ucas, which manages applications to universities and colleges in the UK, confirmed at just before 10am today that part of its site had been unavailable since 8.40am.
Students should have been able to log in to Ucas Track and check whether their preferred university had accepted them. Those that have missed their offers may enter clearing, where applicants are matched to vacant courses.
Clearing is expected to be fraught with 185,000 candidates chasing just 29,000 unfilled places on degree courses.
It comes as the overall A-level pass rate rose for the 29th year in a row, with almost one in 12 exam papers being awarded the elite A* grade.
The number of applicants to UK universities has risen to 673,570, a record high a year before tuition fees rise, and a rise of 1.3% on last year. There were around 487,000 undergraduates accepted at UK universities last year, and there are a similar number of places this year.
Ucas said it had been forced to shut down its Track website temporarily after traffic to the site quadrupled. In a statement, the organisation said: "Traffic to the Ucas Track website is four times the peak per second compared to last year. In order to secure a full service, we have taken the site down for a short time. Full service will be resumed shortly and we apologise for any inconvenience.
"All other Ucas websites, including the clearing vacancy service search function, are still available. Importantly for applicants, the ability to choose a clearing place will not be impacted and this function will open late afternoon as planned."