Rail services are due to return to normal by midday, after engineers made a temporary repair to the signalling at Hemel Hempstead.
The issue was first reported shortly before 8am, and caused delays of between 40 minutes and an hour for London Midland, Southern and Virgin services.
It is understood that two of the four lines to and from London have been affected.
Engineers from Network Rail have made a temporary repair to solve the issue.
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