There are delays of up to half an hour into London this morning and a reduced train service due to a signalling fault.
Great Western Railway trains from Oxford to London Paddington are being delayed following a signalling problem near the capital.
Disruption is expected to continue until 3pm at the earliest and passengers are advised to check before they travel as a reduced service is running at the moment.
National Rail initially said the problem would be resolved by 9am but that time has been put back several times throughout the morning.
Extra stops are being made at Hanwell, which connects to Heathrow Airport.
All train tickets will also be valid on the London Underground.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here