Jenson Button will replace Fernando Alonso for McLaren at Monaco Grand Prix

British former world champion Jenson Button will replace Fernando Alonso at McLaren for the Monaco Grand Prix.

Alonso will miss the race on 28 May to compete in the Indianapolis 500, with the full support of both McLaren and the team's engine partner Honda.

"I'm thrilled to be making a one-off return," said Button, 37, who retired from F1 at the end of last season.

"I couldn't think of a better place to make that return than my adopted home grand prix - Monaco."

The 2009 world champion has spent the winter in California training for Ironman triathlons, his long-time passion.

He signed a contract with McLaren last autumn that committed him to replacing any race driver not able to take part in a grand prix this year.

As part of that contract, the team also has an option to sign him to race in 2018.

Button described Monaco as a "tricky street circuit" but said the McLaren may be "more suited" to the venue than the "faster circuits" Alonso and team-mate Stoffel Vandoorne have driven on so far this season.

"I've won the race before, in 2009, and it's one of my all-time favourite racetracks," he added.

McLaren will be credited with any points won by Button. Both the team and Alonso have yet to win a point this season and are unlikely to be in the shake-up for either title.

Alonso said: "To be honest, if we were fighting for a world championship, we cannot afford to lose a 25-points possibility. Yet we are not in that position, unfortunately."

McLaren racing director Eric Boullier believes Button is fit enough to cope with rigours of racing in Monaco.

"Jenson spent 17 years in F1. He drove these levels of downforce before and, having gone through the differences in technicalities of driving this year's car and last year's car, we agreed that it would be better to spend a couple of days in the simulator than to test in Bahrain on a different circuit in completely different conditions from Monaco.

"He is fit and ready."

Nigel Mansell, another British former world champion, believes Button will do well at Monaco.

Mansell, 63, was the F1 champion when he won the IndyCar series in 1994.

"Jenson is a great world champion and a class act," he told BBC Radio 5 live. "He's probably fitter and more hungry now than he has been for many years.

"He loves Monaco, he's a tremendous driver, and he'll do exceedingly well."

Mansell said the key to succeeding at Monaco was being "incredibly fit and patient", and that Button might "surprise McLaren and himself".

He added: "They are in safe hands with Jenson. He knows the team and they know him very well. The last thing they need is a rookie doing one race, causing problems and crashing the car."

Mansell has no doubt Alonso has the talent to succeed in IndyCars and says he is joining "a great team".

The Spaniard, 35, will race for the Honda-powered Andretti team on 28 May, and the car will be branded a McLaren.

He said he had long held an ambition to win the so-called 'triple crown' of Monaco, the Indy 500 and Le Mans.

"The switch for Alonso will be learning the Indy circuit," said Mansell.

"You have to have the car carefully balanced because if it has any oversteer then it's an accident waiting to happen.

"He will find racing over Indianapolis over 500 miles is fascinating. He will learn a lot very quickly but with the amount of talent he has I wouldn't be surprised if he pulls it off."

Mansell described his time in the United States as a "wild-west experience".

"It's incredible to go across to America and experience that," he said. "It will be fascinating as a racing fan to watch everything unfold next month.

"It's so exciting for racing fans. I think the crossover is wonderful."