Mr Davis added: “Trains from the South West – Devon and Cornwall – will be diverted from Reading towards London Euston – a train an hour.”
GWR said on 27 December an “amended level of train service will run across the GWR network”, with the first services of the day running later than usual.
It said long distance trains would run to and from Reading or Swindon on 28 and 29 December.
A limited, direct service was due to run between London Euston and Plymouth and Penzance.