Jenson Button says he is excited to compete against former team-mate Fernando Alonso at the Le Mans 24 Hours and admits he is "one of the drivers you worry about" when racing - no matter the series.

Both Button and Alonso will make their debuts at the prestigious race in June, at SMP Racing and Toyota respectively, with the Spaniard combining his World Endurance Championship campaign alongside his F1 duties with McLaren.

And 2009 world champion Button, who was Alonso's McLaren team-mate at McLaren for two years before retiring at the end of 2016, has praised the 36-year-old as one of the best drivers in the world.

Alonso wins on WEC debut

"He's definitely one of the drivers you worry about," Button, who starts his WEC campaign at Le Mans, told Sky Sports News' Craig Slater. "Whatever you're racing in, if he's competing - you're going to worry.

"He's not just quick. 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."

And though both drivers will be rookies at Le Mans, Button added: "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.

"It doesn't matter if it's 3 o'clock in the morning or 3 o'clock 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."

Alonso is taking on the WEC challenge as he pursues a career 'Triple Crown' - by clinching the Monaco Grand Prix, which he has won twice, the Indy 500, which he attempted last year, and the 24 Hours of Le Mans.

But even if he wins Le Mans with LMP1 newcomers SMP Racing, Button, who has also been racing in Super GT in Japan this year, doesn't think he'll consider an Indy 500 challenge.

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

Do McLaren regret setting big targets?

When Button left McLaren, the team were two years into a plagued Honda partnership with little sign of light at the end of the tunnel. But while they have now switched to Renault power, F1's 20-time champions are still some way off challenging the sport's 'Big Three', with the team failing to even reach Q3 in qualifying this season.

And Button says the team shouldn't have made such bold claims ahead of their Renault switch.

"The three big teams, it's difficult to challenge them at the moment and it's going to take a while," explained the 38-year-old.

"McLaren came out last year and this year and said they think they might have the best car. It's a tall order to have the best car in Formula 1 when you're racing against Ferrari and Mercedes and Red Bull. And I think we've seen that isn't the case.

"So you'd say the season so far is a disappointment but if they didn't say they had the best car you'd say it's a great season for them. They've made good steps, they're fourth in the constructors' standings, but because of what they said at the start of the year you say maybe it's not what we expected."

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