Briton Froome, 32, finished fourth on Thursday's tough stage 18 and has a 23-second lead over France's Romain Bardet with three stages remaining.

But the fight shown by his rivals means a third Froome victory in a row is not certain, Hayles told BBC Radio 5 live.

"He hasn't won it yet. He should, but this has been a tough battle," he said.

"The way these guys fought up the side of this mountain, they absolutely turned themselves inside out," added the former Olympic track cyclist.

Team Sky's Froome also believes overall victory is not yet certain despite having completed "the toughest part of the Tour".

Following a flat route on Friday's stage 19, the riders face the final individual time trial on Saturday in the penultimate stage in Marseille.

"It's nice to get through the Alps feeling good and looking forward to the time trial now in Marseille," added Froome.

"Rigoberto Uran is my biggest threat in Marseille. From the general classification group, he is the next strongest in time trials. He's only 29 seconds behind so he will be the guy to look out for."

Froome is targeting his first stage victory on the 2017 Tour in Marseille, but says he will "have no regrets" if he rides into Paris wearing the yellow jersey without winning a stage.

"I'll do my best to try to win the stage and ride for the jersey. I've already seen the time trial course. It's a very fast 22km course. I'll do my maximum for sure," he said.

The Briton could become only the seventh man to win the Tour without securing a stage victory, but Hayles disagrees with claims this would be Froome's easiest yellow jersey.

"I've seen a couple of comments saying this is a dull win - this is anything but," said Hayles.

"Everybody tried to put pressure on Chris Froome but ultimately they failed. It didn't happen.

"He has had to try so hard, the margins are so tight - it's not over yet, he hasn't won it yet."

- BBC