It is the only indoor entrance directly connecting the InterContinental Hotel and Bugis Junction shopping mall and had been accessible to the public for 20 years.
Both the mall and hotel were touted as part of a massive "integrated development" when the Urban Redevelopment Authority sold the land in 1990.
But two months ago, the hotel shut its entrance at the mall to the public, in what it said was a move to protect the safety of its guests.
It installed an electronic lock which only those with key cards can unlock and even posted security staff at the entrance.
This means that those moving between buildings will have to make a 300m detour along North Bridge Road and Middle Road.
"The safety and security of our guests is of paramount importance," said the hotel's director of marketing communications and public relations, Ms Ee Jin Lim.
She did not comment on whether there was a particular incident or reason for the entrance's closure.
The hotel, whose lobby is undergoing renovations, said it studied other hotels before implementing the measure. But it did not answer questions on which hotels it had studied or the timing of the move.
Mr Kelvin Chan Teck Heng, the hotel's senior duty manager, noted that the hotel is private property and said it had discussed the closure of the "back entrance" with Bugis Junction operator CapitaLand.
"Would you open the backyard of your house for the public to walk in and out?" he said.
When contacted, CapitaLand would not say whether it supports the hotel's move. The mall and hotel share a common basement carpark with another office building.
Ms Ivy Ang, CapitaLand's general manager at Bugis Junction, said signs will be put up to guide shoppers, who are now unable to get to the basement carpark through the hotel's mall entrance.
The Straits Times visited other integrated hotel and mall developments like Swissotel The Stamford, Marina Mandarin and Mandarin Oriental hotels and found that they do not restrict access to their lobbies through their mall entrances.
The Pan Pacific Singapore has an electronic lock at its mall entrance, but the lock is used only between 11.30pm and 6.30am.
Reporters observing the InterContinental Hotel's mall entrance during weekday lunch hours last week saw security staff turning away more than 20 people. Mr Alfie Ang, 32, a digital marketing manager, said: "I was trying to take a short cut through the hotel but now I have to walk a longer distance. I can understand why, because it's the hotel's property after all. But it is still a mild inconvenience."