An almost full Cathay flight was ordered to steepen its ascent, apparently to avoid unreported military traffic, as it left Shanghai for Hong Kong at the weekend.
Passengers aboard flight CX365 described the pilot telling them of the possible military flight to explain a sudden increase in speed and a sharp right-hand climbing turn.
A Cathay spokeswoman confirmed last night that Shanghai air traffic control ordered the diversion without explanation - a measure she said was 'not uncommon' on the mainland.
News of the order comes amid ongoing talks between international airlines and Chinese aviation authorities to free up more airspace from military control in order to ease worsening congestion, delays and the potential for near-misses.
'It had been a bit bumpy so when the engines went on full blast and the plane lurched to the right I thought we were simply avoiding a storm cloud,' an experienced traveller said on his return to Hong Kong.
'I was amazed when the pilot told us about the possible military plane. I'd never heard anything like it.'
Cathay said the crew saw no approaching plane and the Boeing 747- 400's traffic collision avoidance system showed no traffic. The plane carried 17 crew and 381 passengers.
The Cathay spokeswoman said: 'Later, in his routine updates to the passengers, the captain explained the increase in power in case of passenger discomfort. The climb is a routine manoeuvre.'
One veteran Hong Kong pilot said there was often 'interference' with civil aviation in Chinese airspace. 'Civil air traffic controllers often get late warnings of military flying into civil air lanes and there's nothing much they can do to stop it,' he said.
Another Cathay pilot said such instructions from air traffic control normally came early enough to prevent any abrupt action from the pilots.
'The military flight could have faced some sort of emergency and was forced to divert from its designed path, then ... the air traffic control officers noticed the unusual movement and warned the pilot to re-route its course,' the pilot said.
'Normally we don't even have to inform the passengers because they won't know what the original course is anyway, but if this pilot felt he owed the passengers an explanation, it may indicate that the action he took was too abrupt that he feared the passengers were startled.'
Flights between Beijing and Hong Kong in recent weeks have been severely delayed - in one case by as much as 18 hours - or cancelled, while flights between Shanghai and Hong Kong have been delayed by between 90 minutes and two hours for air traffic control reasons.
The mainland's entire airspace is controlled by the military, with 10 per cent approved for commercial aircraft movements.
As well as talks with the airline body, the International Air Transport Association, the mainland's Civil Aviation Administration is also in active discussions with the military and airports in Hong Kong and Macau to open up more airspace corridors to meet rising future demand.
The then IATA chief executive, Giovanni Bisignani, said in Beijing this year that the key would be releasing more military space for civilian use. 'With size comes responsibility,' he said of China's growth in air traffic.