Two-time Formula One world champion Fernando Alonso labelled Thursday's drain cover incident at the Monaco Grand Prix as unacceptable.

Alonso's McLaren team-mate Jenson Button struck the flying object after it was run over by championship leader Nico Rosberg.

Button suffered significant damage to his front wing and suspension, while Rosberg's Mercedes sustained a puncture after lifting up the cover.

The session had to be stopped, with race officials welding down the cover and inspecting the track for any other potential hazards.

However, Alonso believes there should have been more checks before the F1 cars took to the famous street circuit.

"It's never acceptable when there is an incident like this," the Spaniard said when quizzed about his team-mate.

"It's something that we didn't take care of enough before practice in terms of circuit checks, in terms of the FIA, and everyone has to go into the details to make sure everything is safe to run."

Alonso believes the incident shows why F1 should proceed with intentions to introduce a canopy for the drivers' cockpit.

"This is another reason to come back to the canopy idea," he added.

"Let's make sure that between all of us, between the sport in general, we minimise the risks that are always there."