As many as 100 trains were delayed and at least 20 trains cancelled on the line between London and Edinburgh after a trespasser got on the tracks.
The Network Rail has warned of major disruption until the end of service as people try get to and from Platinum Jubilee celebrations in London.
The trespasser was eventually found hiding in a tree in Hertfordshire.
Trains were stopped while British Transport Police used a helicopter and dog teams to help search for the person in the Potters Bar area, Great Northern Rail said.
It is understood the trespasser avoided capture for some time before being found in the tree.
A Network Rail spokesperson said: “Services from London King’s Cross have been severely disrupted this afternoon owing to a trespass incident in the Potter’s Bar area.
“The individual has been safely removed from the railway and services are up and running again, however, passengers should check the status of their journey before travelling as delays are likely to continue through the afternoon.
“Anyone travelling to an event such as Slam Dunk in Hatfield or to Stevenage for Liam Gallagher should be aware that due to the earlier disruption, a significantly disrupted service is expected on Saturday evening with queuing at stations.”