Two red signals were passed in two days
The rail regulator has asked London Underground to improve its procedures for reporting safety breaches after three train went through red signals.
The incidents were all on the District line in the High Street Kensington area of west London.
They prompted the Office of Rail Regulation (ORR) to write to London Underground (LU) which now has until 15 September to make improvements.
An LU spokeswoman said it was taking the issue very seriously.
The first incident was at West Kensington on 23 February; the second at Earl's Court the following day; and the third on 17 March.
An ORR spokesman said: "A letter the ORR sent to LU relates to three incidents where a train passed a signal at danger in the High Street Kensington area.
"We have requested a plan to address the issues we have raised in the letter by 15 September, which is more to do with how LU manage incidents once a train has passed a signal at danger, rather than the actual passing of signals."
A spokeswoman for LU said: "Safety is our top priority and London Underground has an excellent safety record.
"We take this issue very seriously and are investigating thoroughly. We will respond to the ORR once those investigations have been completed."