Marc Marquez has been denied the chance of fighting for a third MotoGP win in a row after withdrawing from this weekend's Portimao return due to a training accident.

This time it is not the troublesome right arm or shoulder that has kept him on the sidelines but a 'slight' concussion, sustained on an off-road bike, from which he is still feeling 'unwell' several days later.

"This past Saturday Marc Marquez, while preparing for the Algarve Grand Prix with one of his standard off-road training sessions, suffered a fall that caused a slight head concussion," said a Repsol Honda statement.

"After a few days of rest at home and seeing that he was still unwell, today Marquez has been assessed by doctors in a medical check-up to evaluate his current status.

"As a precautionary measure, this coming weekend Marquez will not contest the Algarve Grand Prix."

Marquez had been hoping to use this weekend's event to gauge the improvement in his right arm and shoudler, having made his comeback from last season's injury complications at the earlier April round, when he finished in seventh place, 13s from victory.

After a series of DNFs in the run-up to the summer break, the eight-time world champion took an emotional win at the Sachsenring before returning to the top step in the last two races at COTA and Misano.

That put Marquez in contention for fourth place in the world championship standings, despite missing both Qatar season-openers.

Marquez will now aim to be back on track for next weekend's Valencia season finale.

Repsol Honda is yet to confirm if test rider Stefan Bradl will replace Marquez, alongside Pol Espargaro (who finished second to the #93 in Misano) this weekend.