The Metropolitan line is also affected between Baker Street and Aldgate.
The Hammersmith and City line is experiencing severe delays on the entire line, with Transport for London (TfL) advising those travelling to re-plan their journey.
Delays are affecting the Circle line clockwise and commuters are advised tickets are being accepted on buses.
The Circle line's Twitter account posted a statement which said: "Severe delays clockwise only while we fix a signal failure at King's Cross.
"Tickets are being accepted on local buses"
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