A spokesman could only confirm that the patient in serious condition was a 29-year-old male and was currently at Princess Margaret Hospital.

Wong Wai kin, acting senior scientific officer at the Observatory, said: “Because of a low pressure trough affecting the south east region of Guangdong, we are expecting the region to continue to be affected with this weather over the next couple of days.

“This morning we had an amber warning signal that was cancelled at 11.40am but the landslide and thunderstorm warnings are still in force

“The weather should settle this afternoon and tonight, with some localised rain affecting the eastern territories

“Over the next few days, heavier rain is expected to persist around the coastal region, including Hong Kong."

Last night hailstones the size of golfballs were reported in the New Territories and Kowloon as festivities to mark the end of the Hong Kong Rugby Sevens were washed out.

Thunderstorms were accompanied by near constant flashes of lightning as the Observatory stated that more than 70 millimetres of rain had fallen in some parts in less than an hour.

The Observatory calculated that lightning had struck a total of 2,041 times in the from 9pm, including 482 hits on Hong Kong Island and Kowloon.

A torrent of water cascaded into Festival Walk shopping mall in Kowloon Tong caused by blocked pipes, leaving some shoppers up to their ankles in water. Initially the flood was believed to have been caused by hail smashing windows, but the theory was later dismissed.

One video (see below) appeared to show a huge piece of debris crash to the floor, narrowly missing a shopkeeper.

This morning the centre was open for business as usual, while some shops reported damaged stock.

A spokeswoman for Festival Walk's owner, Maple Tree Greater China Commercial Trust, said "exceptional rainfall" had caused flooding in certain parts of the mall.

In a statement to the Singapore Stock Exchange, the company said it had personnel on site to render assistance and was in the process of assessing the damage caused and making the necessary insurance claims. 

Yesterday evening flooding was reported in areas including Cheung Chau, Tuen Mun, Mong Kok and Sheung Shui, while a landslide was reported in Sai Kung.

MTR stations at Kowloon Tong and Wong Tai Sin also reported flooding.

Paul Kwok, who was having dinner in Tseun Wan when the storm hit said: "It was like the end of the world out there for a while.

"Hail was raining down causing people to run for shelter. It stopped after about 10 minutes."

The Observatory said gusts of wind up to 100kmh had been recorded.

Two people were hospitalised with storm-related injuries, the Hospital Authority said. One was a truck driver hurt when a row of cargo containers toppled onto his lorry.

Sandy Song Man-kuen, a senior scientific officer at the Observatory, said the bad weather was due to a trough of low pressure that brought thunderstorms, hail and rain to coastal areas of Guangdong.

In southern China torrential rain has killed at least 16 people in the last week, bringing chaos to air and road travel.

Heavy rain will continue in Guangdong, Guangxi and parts of Taiwan until at least Wednesday, China’s National Meterological Centre said on Monday. Hailstorms could still occur in southern and central parts of Guangdong, it added.

Hong Kong's last Black Rainstorm warning was issued in May 2013, while the longest black storm on record raged for five hours and 47 minutes in August 1999.

The bad weather came a day after the first big storms of the year soaked crowds at the Hong Kong Rugby Sevens.

Black clouds darkened the stadium before lunchtime Saturday, forcing organisers to switch on floodlights and leading to sodden conditions.

The Observatory said earlier this month that up to seven typhoons were expected to hit the city this year.

It added that as climate change progresses, Hong Kong would see more extreme weather in the future.