There are delays of up to 40 minutes on trains running from Oxfordshire into London Marylebone.
Chiltern Railways says this is due to problems with line side equipment between Beaconsfield and High Wycombe.
Services from Aylesbury in Buckinghamshire are also affected.
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There is a high risk from birch pollen during any good weather. Oak pollen is also on the increase.
Wednesday's weather for the west of the region
Wednesday's weather for the east of the region