Passenger journeys will be "transformed" as the country's oldest rolling stock is set to be replaced with a 21st-century alternative.

Commuters and tourists alike travelling between Portsmouth, Brighton and Lewes will see a noticeable difference to their journeys later this year as the 47-year-old class 313 units are being replaced.

The Argus: The sun sets on the class 313The sun sets on the class 313 (Image: The Argus)

The trains, which do not have toilets or air conditioning, will be replaced by Class 377 Electrostar rolling stock, which feature both toilets and air conditioning.

They are already used on routes across the Southern network, and will be a familiar sight to travellers between Brighton, Eastbourne and London.

The Argus: A class 377 Electrostar leaving the Clayton TunnelA class 377 Electrostar leaving the Clayton Tunnel (Image: The Argus)

Customer services director for Southern Jenny Saunders said: "We’re delighted to be replacing our 47-year-old coastal trains with modern, more reliable and comfortable 21st century rolling stock.

"These will transform passenger travel along the south coast with air-conditioning, comfortable interiors and accessible toilets.

“The busiest service group to Portsmouth will have an extra fourth carriage, compared to operations elsewhere, providing a third more space to meet customer demand on this more congested part of the network.”

The Argus: A class 313 crossing over the railway bridge on the River Adur, on the route to Portsmouth HarbourA class 313 crossing over the railway bridge on the River Adur, on the route to Portsmouth Harbour (Image: Andy Gardner)

The Class 313s, which were built in the British Rail era, were introduced to the Southern Rail network in May 2010 for operation on services on its coastway routes.

13 years later, they are set to be removed from the network from Sunday, May 21.

They hold the title of the oldest electric multiple unit still in mainline operation nationally.

The units garnered much criticism, even from rail union RMT, after they were introduced on the 45 mile route to Portsmouth Harbour. At the time, a spokesman said it was "unacceptable" to have a train without toilets on a route of such length.

The Argus: Unit 313201 is painted in British Rail colours - Pictured passing West Worthing sidings in March 2019Unit 313201 is painted in British Rail colours - Pictured passing West Worthing sidings in March 2019 (Image: Andrew Gardner / The Argus)

The Conservative MP for Lewes, Maria Caulfield, said: "With the addition of air-conditioning, accessible toilets, and a £55m upgrade currently underway, these trains will be much more comfortable for passengers as there will also be more space on board as the three carriage Class 377 Electrostars will be used instead.

"This is welcome news for passengers on the local branch lines who have complained of not having the same level of carriage as the mainline trains for many years and I hope this change will improve local rail journeys.

"Thanks to Govia Thameslink Railway for listening to passengers."