Fortunately the drain cover, dislodged moments earlier by Nico Rosberg's Mercedes, kept low and avoided Button's head in the McLaren cockpit.

"It is one of those things - it hits the car and you think nothing of it," Button said. "But when you actually get back and think about it, if it was another foot or two in the air it could have been a lot worse. We were lucky in a way that it just damaged the car."

Justin Wilson, the British driver who Button competed against in his formative racing years, was killed when a piece of debris struck his head during an IndyCar race in August. Felipe Massa suffered a fractured skull in qualifying for the 2009 Hungarian Grand Prix when he was hit by a spring which fell off a car in front of him.

Button, a staunch advocate for the introduction of improved driver head protection in Formula One, added: "It is not something you expect to happen and we can't have it happen again.

"I heard the drain was welded down so I don't know what more you can do, but they have to stay down.

"We have enough dangers, and a drain cover lifting in the air in front of an open-top car is extremely dangerous."

Button's incident marked one of a number of high-profile incidents ahead of Sunday's blue-riband race at the principality.

Massa crashed at turn one after he lost control of his Williams with just 35 minutes of the opening session gone. British rookie Jolyon Palmer was also in the wall on his grand prix debut at the iconic venue.

His Renault team-mate Kevin Magnussen, the Haas of Romain Grosjean and Manor's Indonesian driver Rio Haryanto also clattered the barriers - the latter two on the exit of the tunnel - while Sebastian Vettel, the four-time world champion, sustained damage to his rear wing following a spin at Mirabeau, before later thudding into the barriers on the exit of Sainte Devote.

On Wednesday, Lewis Hamilton insisted there is no longer any tension between himself and championship leader Nico Rosberg following their opening-lap crash at the Spanish Grand Prix a fortnight ago.

And the Briton, who has not won since October's United States Grand Prix - a losing streak which now stretches to eight races and more than 200 days - laid an early marker by topping the order in opening practice.

But Red Bull's Daniel Ricciardo, who was fastest in the afternoon session by more than half a second, could provide Hamilton with his toughest challenge in Saturday's crucial qualifying session.

"Did we expect Red Bull to be as fast today? Perhaps not, but we shall see on Saturday," Hamilton, 43 points adrift of Rosberg in the championship, said. "They are very much a threat and they are very fast.

"I don't know where six tenths of a second is - I have a little time in me - but it is very, very close."