The work, a critical milestone in the project to deliver electric trains for passengers in South Wales, will see the tunnel closed to trains between 12 September and 21 October.

It will affect the majority of passengers who travel into and out of South Wales including those to and from London, Portsmouth Harbour and south-west England. Passengers traveling to and from London Paddington will be diverted via Gloucester with extended journey times of up to 35 minutes and a reduced frequency of trains. Rail replacement buses will be in operation for all other services.

The work forms part of Network Rail’s £40bn Railway Upgrade Plan to provide a bigger, better, more reliable railway for passengers. As well improved journeys, the electrification of the line between South Wales and London will deliver an economic boost for South Wales thanks to better connectivity to the UK capital, a critical factor for attracting inward investment.

Ahead of the upgrade, a significant amount of preparation work is being undertaken, with four tonnes of soot being removed from the tunnel as well as repairs to the brick work.

During the six-week closure, Network Rail’s orange army will be working all day and night to install over eight miles of conductor rail, designed to provide power to the new electric trains. The scale of the engineering challenge involved and the extensive amount of machinery required to electrify the four mile-long tunnel means that the closure is unavoidable.

Paul McMahon, route managing director for Network Rail Wales, said:

“While this six-week project will result in short-term disruption, there are significant long-term benefits which will come as a result of electrifying the railway including faster, more frequent trains and a boost to economic growth in towns and cities across South Wales thanks to improved connections to and from London.

“Without a solid six-week closure, it would take engineers up to five years to complete the upgrade, causing long-term disruption for passengers and delaying the new electric trains until 2021.

“Wales is open to passengers and freight traffic during the upgrade but we are urging people to check before they travel. Some journeys will take longer and a bus replacement service will be in operation.”

Network Rail is working with Great Western Railway (GWR) to keep passengers informed.

Rob Mullen, GWR General Manager Central said:

“We’re already talking with our season ticket holders and other regular travellers to make them aware of this vital work to modernise the railway between South Wales and London.

“The electrification of the line will enable us to deliver more frequent services, more seats, and to reduce journey times into London by as much as 20 minutes from Swansea. Electrification will also deliver a greener and quieter railway, resulting in cleaner air and a reduction in the noise for those living near the railway.”

“During the work to the Severn Tunnel we will continue to run services between South Wales and London, via a diversionary route, and an extensive bus operation will keep passengers moving between Newport and Bristol. We would like to thank our passengers in advance for their understanding and patience as this vital work is conducted.”

Over the next six months, Network Rail and Great Western Railway will be carrying out an extensive information programme to ensure passengers can make informed travel choices during the temporary closure.

Ahead of the closure, passengers can also keep up to date on journey alterations by visiting: www.GWR.com/severntunnel2016