Virgin Racing suffered acute embarrassment ahead of their qualifying debut for Sunday's Bahrain Grand Prix.

Twenty-two minutes into the final hour-long practice session at the desert-bound Sakhir circuit, the nut on Timo Glock's front-left tyre worked loose. Within a matter of seconds the tyre itself rolled off and along the track before hitting a barrier and coming to rest.

Glock was able to tour back to the garage for the remainder of the lap on three wheels, with the team discovering the problem was a fault with the wheel gun. The device failed to sufficiently tighten up the nuts as mechanics found the other three were also slightly loose.

It was a pity for Glock as he had moments earlier become the first of the newcomers to dip under the two-minute mark for the 6.299km track, clocking a time of one minute 59.245 seconds.

That, though, was still just over five seconds adrift of morning leader Fernando Alonso as Ferrari's new boy warmed up for battle with the hottest practice lap to date.

Alonso set a benchmark of 1min 54.099secs, 1.3 seconds quicker than the best on Friday from Mercedes GP's Nico Rosberg, who was forced to settle for second on Saturday, finishing 0.269secs down on double world champion Alonso.

Red Bull Racing also upped their game as Mark Webber and Sebastian Vettel were third and fifth, sandwiching Michael Schumacher in his Mercedes.

It means for the the third consecutive practice session, the seven-times world champion was beaten by team-mate Rosberg, this time by just under 0.2secs.

Felipe Massa was 0.640secs behind team-mate Alonso, with reigning world champion Jenson Button down in seventh in his McLaren, but nearly a second adrift. Team-mate Lewis Hamilton was a lowly 12th, albeit hampered by a brake duct problem that limited him to just eight laps in comparison to the majority of the leaders who ran up to 16.

In between the two Britons were Renault's Robert Kubica in eighth, the Force Indias of Vitantonio Liuzzi and Adrian Sutil in ninth and 11th, with Williams' Nico Hulkenberg sandwiching the duo in 10th.