A Mass Transit Railway spokeswoman said the delays happened because fewer trains - 22 compared with the usual 31 - were operating on the line, which runs from Tsuen Wan to Central.

The normal two-minute frequency was increased by up to four minutes between Central and Tsuen Wan.

Mongkok and Prince Edward stations were the hardest hit, with delays of up to 10 minutes due to overcrowding. Entrance gates had to be temporarily closed.

The corporation alerted Tsuen Wan line passengers to the delays through station public address systems. Other lines in the MTR network were not affected.

Investigations into the cause of the incident and possible improvements were launched and the MTR Corporation said results were expected within a week.

The Transport Department said it was informed of the fault at 6.15am.

It said it would follow up the case and ask the rail operator to submit a detailed report as soon as possible.

Secretary for Environment, Transport and Works Sarah Liao Sau-tung said the Emergency Transport Co-ordination Centre had monitored the incident and no special transport arrangements had been needed.

She said she would ensure the MTR had adequate and up-to-date preventive measures in place to avoid such mishaps reoccurring.

Dr Liao said she did not think there had been any more MTR faults this year than in previous years. 'The rate [of faults] has been quite normal. But we are not complacent on this. The [MTR] certainly needs to look into what has happened. We asked for an investigation report to make sure they learn from it and take appropriate action to prevent it,' she said.