SINGAPORE - An interfering signal in the Circle Line tunnel is the possible cause of a signalling glitch that has plagued the MRT line this week.

However, more investigation is required into the matter, and further temporary shutdowns of mobile service on the line are possible.

"This interfering signal could be a possible cause of the intermittent loss of train-to-track communications," the SMRT and the Land Transport Authority (LTA) said.

"However, experts have not been able to verify the source."

The signal is "within the operating frequency band of the signalling system", SMRT said in a statement on Saturday (Sept 3) evening.

Mobile signals in four Circle Line stations, Kent Ridge, Haw Par Villa, Pasir Panjang and Labrador Park, were shut down for two hours on Friday evening to investigate the issue.

More tests were also conducted throughout the night between 1 am and 4 am, and investigations are continuing, said SMRT.

However, the tests were inconclusive as there was no recurrence of the signal loss last night. The "intermittent loss of signal communication" has not occurred since 4pm on Friday, SMRT said.

Should and when an incident recurs, telecommunication signals along stretches of the Circle Line will be suspended for short periods of time for further tests.

"If temporary telecommunication signal suspensions have to be carried out, there will be in-train and station announcements along the affected stretches," said SMRT.

"We seek commuters' understanding and patience as we press on in our investigations."

There have been delays on the Circle Line since Monday as a signalling fault caused slower train service, sudden braking and stalled trains.

Drivers have had to be deployed on Circle Line trains, even though the system was designed to be driverless.