McLaren's Fernando Alonso failed a medical and was ruled out of the Bahrain Grand Prix on Thursday with the after-effects of a huge crash in Australia, paving the way for Belgian reserve Stoffel Vandoorne to make his Formula One debut.

The governing International Automobile Federation (FIA) said in a statement that it had been decided the Spaniard could not take part in Sunday's race after an examination at the Sakhir circuit.

"Two sets of chest CT scans were compared and it was decided that there was insufficient resolution of the signs to allow him to compete on safety grounds," it said.

A repeat chest scan had been requested before the Chinese Grand Prix and the results would be analysed before allowing him to race there.

Bahrain is the second race of the 21-round season. The third, in Shanghai, takes place on April 17.

Vandoorne, last year's GP2 champion and a rising star in the sport, is McLaren's official stand-in. The Honda-powered team said he would be at the Bahrain circuit on Friday, in time for first practice.

Alonso's crash in Melbourne, after colliding with Mexican Esteban Gutierrez's Haas in the March 20 season opener, provided a moment of high drama with the race red-flagged before an eventual re-start.

The Spaniard, a double world champion, was fortunate to escape serious injury, climbing out of his wrecked car without assistance and released from the medical centre after precautionary checks.

Alonso had been due to race in Bahrain with a new chassis and replacement power unit after Honda said there was little to salvage.

McLaren revealed this week that the impact in the flying crash had been sufficient to crack Alonso's moulded seat, although they played down the significance of that.

"The fact that the seat cracked but was not broken means it did its job well," said a spokesman. "It flexed helpfully, as it was designed to do, and it efficiently absorbed a lot of the energy of the accident."

Former champions McLaren, who have not won a race since 2012, are fighting back from their worst ever season and appeared to be more competitive in Australia despite failing to score points.

Alonso had missed last year's Australian Grand Prix after a heavy crash in pre-season testing in Barcelona left him suffering from concussion. – Reuters