Kubica emerged virtually unscathed from his 180mph accident

Kubica suffered mild concussion and a sprained ankle last weekend when his car ploughed into a wall head-on.

Doctors from governing body the FIA and the Indianapolis track fear another impact could aggravate those injuries.

Test driver Sebastian Vettel makes his Grand Prix bow in place of Kubica, who faces another check-up next week.

Kubica, who earlier on Thursday said he felt "100% OK", must get medical clearance before the French Grand Prix at Magny-Cours on 1 July.

"I'm disappointed because I felt absolutely ready to race, but I respect the decision," said the Polish driver.

I am looking forward to my first F1 race but I wish it had come under different circumstances Sebastien Vettel

"It was made because there is too much risk to let me race in this Grand Prix in case I have another impact.

"I will go home now. I wish Sebastian all the best. It is good for him and I am pretty sure he will do a good job."

Kubica's car launched into the air and plunged into the wall after being flipped into the air by Jarno Trulli's Toyota.

He was lucky to escape without further injury especially as his feet could be seen through the sheared off front end of the vehicle.

BMW have drafted in Timo Glock as the nominated reserve driver as Vettel, 19, prepares to make his debut.

"I am looking forward to my first F1 race but I wish it had come under different circumstances," said the German driver.