Having partnered Fernando Alonso for two full seasons at McLaren, Jenson Button knows how the Spaniard ticks and what a formidable talent he is.

As he prepares to compete against his former teammate at Le Mans, the Briton sent out a warning to rivals never to discount the two-time world champion, no matter what he's driving.

"He's definitely one of the drivers you worry about," the Briton told Sky Sports News. "Whatever you're racing in, if he's competing, you're going to worry.

"He's not just quick," he continued. "He's a very clever driver, he's got the experience and he is always there.

"If you have a great day, you might out-qualify him and you might have a great race but he will always be on your heels. He very rarely has a bad race, which is tough when you're team-mates with him.

"We're going to go into Le Mans racing against each other and I know if we're in a stint together, he's not going to put a foot wrong," he admits. "It doesn't matter if it's three in the morning or three in the afternoon, he's going to be 100 per cent.

"You've got to keep up with that and it's tough but that's what I love about racing against the best drivers in the world. They keep you on your toes."

Having won at Monaco, a win at Le Mans would put Button in sight of the same triple-crown that Alonso is chasing. However, the Briton admits that he has no ambition to contest the Indy 500.

"I'm going to leave that one for Fernando to do," he said. "This is pretty much the busiest season I've had in my career."