The 130-year-old structure will be closed for six weeks from Monday, 12 September.
The project will see engineers prepare the tunnel for electrification, which will deliver more seats, more frequent services and more energy efficient journeys.
The scheme is part of Network Rail's £40 billion Railway Upgrade Plan which will result in the railway to Cardiff becoming electrified by 2019.
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Dan Tipper, area director for Network Rail Wales, said: "This iconic project will result in short-term disruption for passengers and we would like to thank everyone for their understanding and patience while we complete this essential upgrade.
"There are significant long-term benefits which will come as a result of electrifying the railway to Cardiff by 2019 including faster, greener, more frequent and reliable journeys for passengers. It will also provide a boost to economic growth in South Wales thanks to better connectivity to and from London, a critical factor for attracting inward investment.
"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 electrification of the railway until 2021."
(LM/CD)
