By Andrew Benson
BBC Sport at Silverstone
Championship contender Kimi Raikkonen will be penalised 10 places on the grid at the British Grand Prix after an engine failure in Saturday practice.
The Finn's McLaren-Mercedes stopped on his first flying lap of the second part of the final session before qualifying.
It is a major blow to his attempts to overhaul Renault driver Fernando Alonso's 24-point championship lead.
Alonso set the practice pace with a lap of one minute 20.077 seconds, beating McLaren's Juan Montoya by 0.051secs.
Qualifying, which sets grid positions for Sunday's race, is at 1300 BST.
Toyota driver Jarno Trulli was third fastest in practice, 0.372secs behind Alonso and just ahead of team-mate Ralf Schumacher.
Jenson Button was fifth in his BAR, while the other British driver David Coulthard was 16th overall after also stopping out on the track midway through the session.
Renault's Giancarlo Fisichella, Michael Schumacher's Ferrari and the Williams of Mark Webber were sixth, seventh and eighth.
Webber's lap late in the session was the first vague hint of a return to form for Williams after a dreadful race in France last weekend.
Williams, whose engine partner BMW is to take over the Sauber team next season, introduced a major update to their car in France, but it was a failure.
The once-dominant team are running an old-specification car alongside a revised one at Silverstone in a bid to establish which one is faster.
Raikkonen had set the pace in the first part of Saturday's split session, and was on a faster lap still when his McLaren ground to a halt at Club corner on his first flying lap of the second session.
It is the second time in succession that Raikkonen was left to count the cost of engine failure after being relegated 10 places to 13th at last week's French Grand Prix.
Follow the British Grand Prix live on the BBC Sport website. Qualifying is at 1300 BST on Saturday 9 July, with the race at the same time on Sunday.
Final practice leaders:
1. Fernando Alonso (Spa) Renault 1:20.077
2. Juan Pablo Montoya (Col) McLaren 1:20.128
3. Jarno Trulli (Ita) Toyota 1:20.449
4. Ralf Schumacher (Ger) Toyota 1:20.524
5. Jenson Button (GB) BAR 1:20.763
6. Giancarlo Fisichella (Ita) Renault 1:20.798
7. Michael Schumacher (Ger) Ferrari 1:20.893
8. Mark Webber (Aus) Williams 1:21.004
9. Nick Heidfeld (Ger) Williams 1:21.052
10. Christian Klien (Aut) Red Bull 1:21.064
11. Rubens Barrichello (Bra) Ferrari 1:21.371
12. Jacques Villeneuve (Can) Sauber 1:21.476
13. Felipe Massa (Bra) Sauber 1:21.704
14. Takuma Sato (Jap) BAR 1:21.931
15. Tiago Monteiro (Por) Jordan 1:22.036
16. Narain Karthikeyan (Ind) Jordan 1:22.634
17. David Coulthard (GB) Red Bull 1:23.165
18. Christijan Albers (Net) Minardi 1:23.444
19. Patrick Friesacher (Aut) Minardi 1:23.586
20. Kimi Raikkonen (Fin) McLaren