Modern electric trains have been turning into 'steam trains' during hot weather because the sophisticated air conditioning does not work.

After a long day at work, commuters have complained of gasping for fresh air and sweating profusely when they leave the carriages.

South Central said the air conditioning on its new modern trains was tripping out because of the recent high temperatures and it was working to resolve the problem.

But it has led to the new Electrostar trains, which are gradually being introduced throughout the South Central franchise, including the Brighton Line to and from Victoria, being dubbed Electrosaunas by passengers.

The situation has been made worse because only a few hopper windows can be opened, only by using a key held by the train guard.

The Brighton Line Commuters, the pressure group representing Sussex commuters, has asked South Central to look into the problem as a matter of urgency.

Chairman Shelley Atlas said: "It is a real problem. We are getting reports all the time about whole trains or certain carriages without the air conditioning working properly.

"Recently two main evening commuter trains arrived at Brighton from Victoria and London Bridge with passengers complaining the air conditioning was not working. One woman was feeling really unwell. We are taking the matter up with South Central as a matter of urgency."

The new Electrostar trains are being made at the Bombardier works in Derby in an £856 million order to upgrade the fleet, one of the biggest orders ever placed by a rail company in Britain.

A total of 182 new trains, making up 700 carriages, have been ordered.

Rail campaigner John Stanaway, of the Southern England Coastway Rail Future, is also concerned about the air conditioning.

He said: "I got off one train and was soaked through. Rail companies have got to ensure the air conditioning on trains with fixed windows is working properly."

Design consultant, Henry Law, a regular traveller, said: "On trains with air conditioning you need an air lock to keep it working. The air conditioning systems cannot cope because immediately the centre doors open, the systems are trying to air condition the entire platform."

Samantha Hodder, head of communications for South Central, said: "The problem with the air conditioning is that it has been tripping out because of the recent high temperatures, which has forced the air conditioning to work overtime. We are working with Bombardier to get the matter resolved.

"These are modern new trains which have gone through an extensive design process and they have been well received by our customers."