SINGAPORE - About a quarter of the 1.8 million users of TraceTogether have updated their app to the latest version, which requires them to re-register with their identification numbers.

These 450,000 app users had previously entered their mobile phone numbers, but the new requirement to supply more personal data is needed under stepped up measures to combat the spread of Covid-19.

A spokesman for the Government Technology Agency (GovTech) told The Straits Times that all versions of the app will still work in exchanging and logging Bluetooth signals with nearby devices.

In the event that someone is infected with Covid-19, the Bluetooth data logs in the devices will quickly unravel who had been in close contact with the patient.

GovTech said that making app users re-register with their NRIC, FIN or passport numbers will allow close contacts to be identified more accurately and quickly.

For instance, people may have multiple mobile numbers registered in their names but their family members or workers may be the ones using the numbers.

"So, for example, Mr A may own several mobile telephones all registered under him but then he gave them to different workers. Tracing by just the mobile number is problematic," said Mr K.K. Lim, head of cyber security, privacy and data protection at law firm Eversheds Harry Elias.

The updated app also carries a unique NRIC/FIN barcode that belongs to the user, enabling quicker check-in at stores, buildings and locations with the national digital check-in system SafeEntry.

GovTech urged users not to delete the old version of their TraceTogether app, as it would result in a loss of previously collected data. Rather, they should update their existing app with the latest version.

The 1.8 million users who have downloaded TraceTogether represent about 25 per cent of Singapore's population - still short of the minimum of 75 per cent needed for TraceTogether to work well.