The line currently runs from Swansea, through Cardiff, before going on to Reading and London Paddington, but under new plans, rail passengers travelling from west of Reading will no longer have to double-back on themselves via Paddington.
The new link means passengers in Cardiff and Swansea could have their travel times to Heathrow cut by up to 45 minutes and 53 minutes respectively, while those in Reading could have up to 27 minutes shaved off their journey.
The link will allow businesses to connect directly with Heathrow, which the UK's busiest airport, says would open up new opportunities for trade abroad and make it faster for leisure travellers to catch flights.
Colin Matthews, Heathrow’s chief executive, says: “Network Rail’s plans will drive economic growth through improved access to overseas markets. Heathrow is already in a better location than any other UK airport.
“Western Rail access will build on that strength, helping to make sure that the whole of the UK benefits from the growth opportunities global connectivity brings.”