The website had been updated over the weekend by the ICA in preparation for the surge in applications as vaccinated travel lanes (VTLs) between the island republic and its two closest neighbours, Malaysia and Indonesia, were due to open next Monday.

Nearly 35,000 users tried to log in at the same time but were unsuccessful, with the ICA blaming it on an “error in configuration”, Straits Times reported.

The website was restored at around 3pm. However, users were advised to log in later, due to the high volume of traffic still trying to get access.

Under the VTL for travel via the Causeway and Second Link, Singapore is only allowing long-term pass holders and short-term foreign travellers to apply for the vaccinated travel passes.

“However, there is no quota or restrictions for vaccinated travel passes for air travel under the VTL. Singapore citizens and permanent residents do not need to apply to enter,” the ICA was reported as saying.

With the VTL, travellers between Singapore and Malaysia need not undergo any quarantine.

Singapore and Malaysia have strong business ties with more than 10,000 Malaysians living and working in Singapore.

But since Malaysia closed its borders last March, families on both sides of the border have been separated for nearly 20 months.

Before the pandemic, the Singapore-Kuala Lumpur air route was the busiest international air route in the world, recording about 40 flights and an average of 7,000 arrivals daily at Changi Airport.