A spokeswoman for Network Rail said: "This upgrade to the Severn Tunnel was planned more than a year ago and we have been working with the train companies to ensure that the disruption is minimal.
"Many trains are being directed via Gloucester and it might take a bit longer but we should get everyone to London."
A First Great Western spokesman said: "We don't yet know what day the game will be on or what time the kick off will be, but as soon as we know these details we will be working on getting extra trains on so that people from Cardiff can get to Wembley."
Please check when booking that you have enough time to get to and from London.