Thursday, 18 January 2018:

Adelaide's heatwave continues to cut a swathe through the Tour Down Under, with a community ride cancelled and stage four starting an hour early.

Organisers have confirmed that Friday's Challenge Tour ride will not go ahead, because of the forecast 40-plus temperature.

They will also start stage four, a tough race from suburban Norwood through the Adelaide Hills to Uraidla, at 10.30am rather than 11.30.

Thursday's family ride at Victor Harbor was also cancelled because of the heatwave and stage three was shortened by 26km.

The temperature at Victor Harbor is normally a few degrees cooler than in the city, but at noon it had soared to 42.

The Challenge Tour is one of the key features of the Tour week in Adelaide, with more than 5000 riders entered.

All will receive refunds.

The ride route follows the same course for stage four of the Tour, with the challenge starting a few hours before the race.

But given there is no forecast let up of the 40-plus furnace until Saturday, Events SA chief executive Hitaf Rasheed said they had followed the event's extreme weather plan.

"We take the safety and welfare of Challenge Tour participants very seriously, and it is their wellbeing that comes first," she said.

"Whilst we know there will be some disappointed participants, and we too are disappointed to be making this decision, we are confident this is the right decision.

"Safety of all involved is the highest priority and won't be compromised."

Tour race director Mike Turtur changed the stage start time to ease the heat stress on the riders.

"It is the first time in the history of the Challenge Tour that we have been required to cancel and it is the right thing to do," he said.

"The cancellation of the ride means that we are able to start stage four one hour earlier."

AAP