The work will be carried out between Saturday, May 23 and Monday, June 8.
This will impact routes between London Paddington and Cardiff Central, Swansea, Carmarthen, and Pembroke Dock, as well as services between Bristol Temple Meads, Portsmouth Harbour, and Cardiff Central.
Due to the tunnel closure, trains between London Paddington and Swansea, and some services extending to and from Carmarthen and Pembroke Dock, will be diverted between Swindon and Newport via Gloucester.
Trains will depart earlier than usual from London Paddington, Reading, and Swindon, and will not stop at Bristol Parkway.
People using Bristol Parkway are being advised to travel via Bristol Temple Meads for connecting rail services.
Some services between Bristol Temple Meads and London Paddington will arrive and depart Swindon and further stops a few minutes later than scheduled.
Services between Cardiff Central and Portsmouth Harbour will begin or end at Bristol Parkway and will rejoin the normal route at Filton Abbey Wood.
Most trains between Cardiff Central and Bristol Temple Meads will operate instead between Bristol Parkway and Bristol Temple Meads.
Trains between Cardiff Central and Taunton or Exeter St Davids will start or terminate at Bristol Temple Meads.
Passengers are advised to use journey planners to check connections to and from Cardiff Central, including rail options via Gloucester.
Local stopping services between Filton Abbey Wood and Bristol Temple Meads will not operate during the closure.
Most Bristol Parkway to Portsmouth Harbour services will include additional stops at Ashley Down, Stapleton Road, and Lawrence Hill.
Replacement buses will operate between Bristol Parkway and Newport.
Fast services will run twice per hour, taking approximately 50 minutes.
A stopping service will also operate once per hour, calling at Severn Tunnel Junction and Patchway.
Late-night replacement buses from Bristol Parkway will drop off only at Newport and continue through to Cardiff Central.
Rail tickets will be accepted at no extra cost on alternative transport services, including M4 Metrobus routes between Bristol Parkway and Patchway, and the T7 TrawsCymru service between Bristol and Chepstow bus stations.
Passengers are urged to check their journeys in advance using the National Rail Enquiries journey planner or the Traveline website for bus routes.
Further guidance on using replacement bus services, including accessibility and information on travelling with bicycles, can be found on the National Rail website.
Pick-up and drop-off locations for replacement buses can be found by checking station signage or by searching for the station on the National Rail website.
Passengers are reminded that replacement buses may be busier than normal, and additional journey time should be allowed.
