The Montreal marathon that was set to take place this weekend has been cancelled due to unseasonably hot temperatures.

The full 42.2km race is cancelled, while the half-marathon will begin an hour early.

The forecast is showing a record high of 30 degrees Celsius on Sunday, with the Humidex levels making it feel like 39.

"In consultation with medical services officials and our city partners, event organizers have determined that the unseasonably warm weather conditions are not suitable to provide a safe race experience for the 42.2 km distance. The full marathon will therefore not take place," a news alert read on the Rock N' Roll Oasis website.

Race organizer Louis Malafarina said he knew the 5,000 people who spent months training for the race would be disappointed.

"We're making a decision for 30,000 participants. 5,000 participants for the marathon itself, but the reality is not all 5,000 are professional runners who have done 12, 13, 14 marathons under any conditions," said Malafarina.

He pointed out in such extreme weather, runners need to know the medical demands could easily overwhelm staff.

"A number of them are giving it a shot for the first time and we need to communicate with them that there are conditions under which it's not right to attempt a marathon and certainly under these conditions it's not right," said Malafarina.

The marathon has never before been cancelled, and organizers spent 24 hours debating what to do because of the heat and humidity, though there have also been grueling conditions in the past.

Race medical coordinator Eddy Afram said that in 2014, they had 1,000 runners who needed to be treated for some form of heat exhaustion and they simply don't have enough personnel to deal with potentially 1,000s more.

“We've increased our resources since then. We are always increasing our resources, but there's also the impact of the event on the public health system and our responsibility is to prevent that,” said Afram.

The half marathon will begin Sunday morning at 7:30 a.m., and will have a time limit of three hours. Metro trains will begin running at 5:30 a.m. in Sunday so that half-marathon and 10k runners can arrive at the start line in time.

Marathon runners can choose to run the half marathon, defer their race to any city or to the Montreal 2018 race, or receive a full refund.

Organizers say they expect to take a financial hit due to the cancellation, but still expect more than 20,000 runners to take part in this weekend's event.

More info here: