Fernando Alonso should not have been denied his spot on the grid for today's Bahrain Grand Prix, McLaren chairman Ron Dennis has said.

Dennis has campaigned on behalf of the two-time world champion, declared medically unfit by the FIA on Thursday following his Melbourne horror crash, to be allowed to race.

But Formula One's governing body is refusing to budge on its initial diagnosis meaning Stoffel Vandoorne, McLaren's reserve driver, will make his grand prix bow later on Sunday.

"The question is would Fernando have been a danger to other drivers?," said Dennis on Saturday night.

"And after that the question is really one of assessment and if you've got a cracked rib and you want to drive with it, it's your business isn't it?

"Virtually every team sport in the world, the fitness of the athlete, the football player, the ice-hockey player, the skier, is determined by the team.

"The position of the FIA, I feel, is to ensure the safety of the other drivers. If a driver feels a bit of pain, and wants to drive with a bit of pain, I think that's the driver's decision."

Alonso sustained fractured ribs in his enormous shunt at the season-opening race in Australia a fortnight ago.

He flew to Bahrain confident of being given the all-clear to race, but was prevented from competing on safety grounds after a 90-minute examination on Thursday.

Dennis added: "The two sets of doctors in Spain had cleared Fernando to drive and fly, so we were very surprised to get a different interpretation here.

"We got through yesterday, and Fernando was feeling aggrieved as he felt so good that he felt he wanted to drive.

"So not to be able to re-evaluate the situation this morning I didn't feel was very appropriate."

Vandoorne, the 24-year-old Belgian, will compete instead of Alonso, and he will start his first-ever Formula One race ahead of Jenson Button.

Despite impressing in practice on Friday, Button could manage only 14th in qualifying, with Vandoorne two places further up the grid.