A signalling fault between London Blackfriars and London St Pancras International means Thameslink can run fewer trains until it is fixed.
This causing cancellations and delays of up to 10 minutes to or from London St Pancras International and West Hampstead.
Other routes may take 30 minutes longer than usual.
Affected services include those between:
- Brighton / Three Bridges / Gatwick Airport / East Grinstead and Bedford
- Brighton and Cambridge
- Horsham and Bedford / Peterborough
- Three Bridges and Flitwick / Peterborough
- Sevenoaks / Orpington / Rainham and Luton
- Sevenoaks and Welwyn Garden City
- Orpington / Rainham / Sutton (Surrey) and West Hampstead Thameslink
- Sutton (Surrey) and St Albans City