The MTR on Wednesday said a section of railway track near the Causeway Bay station will be replaced after services end at night, following the discovery of cracks during an overnight inspection.
The rail operator said the section in question had been reinforced for now and engineers had been deployed on site to monitor the situation.
A spokesman stressed locks had been put in place to ensure the track will not dislocate and trains run safely.
Trains journeys on the Island line took three to five minutes longer than usual in the morning, as trains had to be slowed when passing between Wan Chai and Causeway Bay.
Services had since returned to normal, but the MTR said train frequency may be adjusted in the evening again.