Network Rail said that there was a small slippage by the track yesterday, which caused speed restrictions. At 10.10am today, it was noticed that the situation had deteriorated and the landslide covered the downward line. By 11 the upward line was also blocked.
As well as blocking the line, the landslide has caused damage to signalling equipment and track circuit has been damaged. It is thought that heavy rains caused the slippage.
The landslide happened near Chipping Sodbury, north east of Bristol.
First Great Western have said that the line between Swindon and Bristol Parkway is expected to remain closed until Saturday. Services to and from South Wales will be diverted, with extended journey times of up to 30 minutes.