Airlines in the greater Pearl River Delta are expected to be carrying 240 million passengers a year by 2030.

But the move could also be a deciding factor in winning approval for the new runway at Hong Kong International Airport.

Without such an agreement, it would be impossible to fully utilise the HK$136.2 billion runway - a fact pointed out by the Airport Authority in its technical report.

It states that 'to fully realise the potential capacity gain of a third runway, the Pearl River Delta airspace will need to be redesigned to be able to provide ... a northern circuit at the Hong Kong International Airport, long final approach tracks and independent arrival procedures'.

The Civil Aviation Department said the three cities have agreed a series of measures to optimise the airspace structure and have agreed to jointly plan any future changes in airspace, air traffic management standards and flight procedures.

While this will be welcomed by airports and carriers in the region, some critics have warned that without a clearly-written commitment, such a 'consensus' may not materialise - as past experience has shown.

Albert Lai Kwong-tak, of the Professional Commons - a community group that has campaigned against major infrastructure projects - said: 'When Hong Kong bought a stake in the Zhuhai airport back in 2006 it was alleged that the mainland airspace will be rationalised. It never happened.'

Academics have also repeatedly questioned if the region's congested airspace could accommodate the traffic growth generated by a third runway. A person familiar with the situation said the Civil Aviation Administration of China has agreed 'in principle' to obscure the airspace boundary that divides Hong Kong and Shenzhen by 2020.

A former civil aviation officer explained that this would mean an extension of the 'horizontal' boundary, with the 'vertical' boundary being maintained.

He said: 'Hong Kong aircrafts can enter Shenzhen airspace as if they were still in local air, but that freedom is confined within a certain height.

'It is like segmenting the sky into different storeys, with the third floor operated by Hong Kong and the fourth floor by China.'

And the so-called 'third floor' will not extend indefinitely into the mainland. At some points, the aircraft would have to climb back into 'the mainland floor'.

But such an arrangement would offer much greater flexibility than existing arrangements, where aircraft can enter a different airspace only at fixed handover points.

'The handover points are usually located above airports, where traffic is busy,' the person said.

'[The new arrangement] could greatly increase the number of cut-in points for flights, which means the handover procedures could be conducted at much less congested airspace.'

Macau authorities were also said to have agreed to share airspace with Hong Kong to ensure the proposed third runway has a long-enough approaching track for incoming flights.

Peter Lok Kung-nam, former director-general of civil aviation, said such measures would help boost airport capacity.

But he asked: 'Have they signed any memorandum yet? If not, these solutions could still very much be a plain concept, and any decision that is made on the [third runway] project based on 'promises' is shaky.'

Sources close to the talks said they were very confident the opening up of airspace would be confirmed, because it is as important for mainland airports as it is for Hong Kong.

One said: 'It's not just about Hong Kong. Shenzhen's second and third runway cannot be fully utilised either without optimisation of the airspace. And the mainland has some 200 airports - they have much bigger interest in this than us.'

The Airport Authority says the 4,000-metre third runway at Chek Lap Kok is needed because the airport will reach full capacity by 2020.

Last year Chek Lap Kok handled 306,500 flights, 50.9 million passengers and 4.1 million tonnes of cargo.

With the third runway, the airport could cope with 620,000 flights, 97 million passengers and 8.9 million tonnes of cargo each year, the authority says. The public consultation period will end on September 2.

102

The number of flights per hour that could be handled at Chek Lap Kok with the third runway. The current number is 68