Flooding on the train tracks is causing delays to trains through St Albans and Radlett.
There is up to 25 minute delays on trains running from London St Pancras through to Luton due to flooding between Radlett and West Hampstead.
One of the southbound tracks currently remains blocked.
Thameslink have said on their website that there is no firm estimate yet of how long the disruption will last, but it is likely to continue for some time.
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