Amisha Hira, 26, said she noticed it was unusually hot in the aircraft soon after boarding flight CX255 at about midnight.

'There was hot air coming out of the vents,' she said. 'The captain announced that it might be because the engine was off and that it should cool down once we were up in the air.'

The plane, carrying 222 passengers, took off 22 minutes late.

'After two hours in the air, I felt really claustrophobic and had an anxiety attack. I told my husband I felt like I couldn't breathe,' frequent flier Mrs Hira said. 'An air hostess noticed and they were fanning me, getting me napkins with ice every 10 to 15 minutes to calm me down.

'My driver in economy class told me the situation was even worse there, with at least two people lying down in the walkway.'

Mrs Hira said she noticed several economy class passengers, including two elderly women, seated in the crew area being fanned by staff. Others used damp towels and mist sprays to counter the hot air being pumped out.

A Cathay Pacific spokeswoman said a valve in one of the plane's air-conditioning systems was faulty.

The flight turned back to Hong Kong at 2.52am, two hours after takeoff, and landed at Chek Lap Kok at 4.26am. Passengers boarded a replacement plane at 9am.

'The valve has been replaced and the aircraft has returned to service,' the spokeswoman said. 'We apologise for the inconvenience and discomfort caused.'

However, Mrs Hira asked why passengers were made to put up with such difficult conditions for four hours when the problem was apparent from the outset.

'If your parts are faulty, you should not take off to begin with. If they thought it would get better but it didn't, it shouldn't have taken them so long to decide to turn around - it was a 12-hour flight.'

The Civil Aviation Department is awaiting a full report on the incident from the airline's engineers.