The much-anticipated air travel bubble to allow quarantine-free travel between Singapore and Hong Kong is set to take off on May 26 - six months after its initial planned launch.

Singapore's Ministry of Transport said yesterday that the travel bubble will have a cautious start. There will be one flight a day in each direction, capped at 200 passengers on each flight for the first two weeks.

This will then be increased to two flights per day from June 10.

Passengers from both sides must test negative for Covid-19 before boarding the direct flights, and take a test on arrival as well.

Those using Hong Kong passports leaving the territory are also required to be fully vaccinated before taking the bubble flights. This will not apply to Singapore citizens and others using non-Hong Kong travel documents for departure.

On whether it is a good time to start the bubble amid reports of Covid-19 community cases in recent weeks, Transport Minister Ong Ye Kung said yesterday: "You can't say for sure that everything will be smooth-sailing... I think that is the nature of this pandemic."

Referencing the Australia-New Zealand air travel bubble which was partially suspended last week five days after its opening, Mr Ong said this kind of stop-start situation is going to be the norm. But he said the improved Covid-19 situation in Hong Kong meant conditions are right for the bubble.

The bubble will be suspended for at least 14 days if the seven-day moving average of the daily unlinked Covid-19 cases exceeds five in either city.

It can resume only when the seven-day moving average of the daily number of unlinked Covid-19 cases falls below five.

This must be followed by three consecutive days where the daily number of unlinked local cases does not exceed three in either city. Also, on the last of these three days, the seven-day moving average of the daily number of unlinked local Covid-19 cases must not exceed five in both places.

The bubble was slated to start last November, but was deferred because of the worsening Covid-19 situation in Hong Kong then.

Asked why the cities plan to start the bubble only next month, Mr Ong said it is to give the industry as well as travellers time to get ready.

He added: "We want to take that one month to make sure that all the control measures we have put in place in both cities will work - and I believe they will."

In Hong Kong, Secretary for Commerce and Economic Development Edward Yau said the key control element under the arrangement is to have pre-departure and arrival tests, when asked why Singaporeans flying to Hong Kong are not required to be vaccinated.

"We have made provisions for the entire community to be vaccinated so as to protect themselves, and the purpose of asking Hong Kong residents going out to have vaccination prior to joining the scheme is in fact protecting their own health," he said. "This would be an additional safeguard for our people."

Singapore Airlines said the first bubble flight to Hong Kong, SQ882, will depart on May 26 at 8.40am.

Meanwhile, Cathay Pacific said its first bubble flight, CX759, will depart from Hong Kong for Singapore at 9.10am on May 26.

The travel bubble will be the first of its kind for both cities.

It will be the third one in the world after similar arrangements between Taiwan and Palau, as well as Australia and New Zealand.

Mr Ong said: "I think the Hong Kong-Singapore bubble is still quite unique in the sense that it is between two cities which are aviation hubs as well as financial services hubs.

"So, to get this bubble up successfully, I think, will have a significant signalling effect to the rest of the world."