Sainz led McLaren duo Oscar Piastri and Lando Norris, with the near-constant rainfall preventing the field from setting representative times at the Spa-Francorchamps circuit.

With rain forecast to continue throughout the day, the FIA confirmed that times from the session would not be used to set the grid for Sunday's race in the event that Qualifying, which is scheduled to start at 4pm, was unable to be completed. The grid would instead be set by the current order of the drivers' standings.

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The sport's governing body added that "other sessions", which appears to be a reference to the Sprint Shootout that precedes Saturday's shortened-distance race, could be cancelled or rescheduled to enable Qualifying to take place.

With F1 visiting Spa just four weeks after racing driver Dilano van 't Hoff died in an accident in the Formula Regional European Championship at the circuit, there was much discussion in the paddock on Thursday about the safety of the track in wet conditions.

The rain that had been forecast for Friday appeared to be manageable as the session began but quickly intensified, with Alex Albon and Yuki Tsunoda both highlighting the difficulty of the conditions as the pair ran off track.

Those moments led to the field returning to the pit lane for a short period, before Sainz went out on track to trigger a brief spell of action.

The Ferrari driver took advantage of the rain relenting to move to the top of the timesheet with a 2:03.207, but a stronger downpour began to ensure that only Piastri was able to finish within a second of the leading time, with Norris a further few tenths back.

The next fastest car was the other Ferrari of Charles Leclerc, who was almost five seconds back from his team-mate's effort.

World championship leader Max Verstappen, who is seeking an eighth successive Grand Prix victory this weekend, was bottom of the timesheet after not setting a lap-time.

The Dutchman, who is also attempting to help Red Bull extend their record 12-race winning streak, has incurred a five-place grid penalty for exceeding his gearbox allowance for the season.

As conditions grew more difficult, another series of incidents followed, with Alfa Romeo's Zhou Guanyu tapping the barrier and Albon running onto the grass, before Sargeant was completely caught out.

The American rookie lost control of his Williams at the end of the Kemmel Straight, slamming into the barrier and bringing out a red flag, which paused the session for 10 minutes.

With the rain continuing to fall heavily, there was little action when the track was cleared for action with 20 minutes remaining, with only a few drivers opting to return to the tarmac for practice starts in the closing minutes.

While the limited running done in practice meant that the limited visibility caused by the rain wasn't a major issue, a far more busy track in Qualifying will leave race director Niels Wittich under pressure to protect the drivers' safety.

Mercedes driver George Russell, who finished a place behind team-mate Lewis Hamilton in 13th, on Thursday said the FIA must make "bold decisions" to protect the drivers.