Latest news on missing tourists in Greece: Four more dead and two missing in extreme heat after Michael Mosley tragedy

Tourist attractions in sweltering Athens and elsewhere were forced to close on Wednesday and Thursday, with the Acropolis closing its doors to visitors after reports of people fainting while waiting in line.

Greek authorities issued a level three heat warning and sent automated alerts to mobile phones urging people to work from home and avoid strenuous outdoor activities.

The highest temperature was recorded in Chania, a city on the northwest coast of Crete. On Thursday, the thermometer reached 44.5 degrees. On Friday morning, the area was also shaken by an earthquake measuring 3.7 on the Richter scale.

Greece, a popular holiday destination in much of Europe, has now been hit by the longest heatwave ever recorded. A heatwave is defined as temperatures above 38 degrees Celsius that last for at least three days.

Greek state television meteorologist Panos Giannopoulos said the country had experienced its earliest heatwave ever: “In the 20th century, we never had a heatwave before June 19. In the 21st century, we had several, but none before June 15.”

One of the two French women missing on the Greek island of Sikinos sent a picture to the owner of her hotel in which she said she had fallen and was not feeling well, the mayor said.

However, Greek authorities failed to locate the location of her mobile phone signal and concluded that she was in the south of the island.

The message, sent on Friday morning, was accompanied by a photo in which the woman appeared to be lying on the floor, reports the French newspaper South-East reported.

Rescue workers fear the missing tourists are trapped in an area of ​​the remote island – with a population of about 400 people – that is neither easily visible nor mapped for rescue workers, Mayor Vassilis Marakis told broadcaster ANT1.

“The day they went hiking it was extremely hot and today, as we work to locate them, there is a strong wind blowing,” Mr Marakis was quoted as saying. “Some of the trails on our island are mapped, but others are not.”

“We don’t even know if the two hikers were travelling together or separately. However, we are continuously combing the area and firefighters from Syros and volunteers are participating in the investigation.”

About a dozen people arrived from Santorini on Sunday to help with the search, armed with two drones.

Greece is quantifying the cost of an early heatwave that killed several tourists and offers a taste of the weather extremes likely to await the country this summer.

An urgent warning has been issued to all travellers to the country as four holidaymakers have died and others are missing in record temperatures.

my colleague Rich Booth Reports from Greece: