Monaco GP: Sebastian Vettel top as Jenson Button given penalty
Sebastian Vettel set a blistering pace as Ferrari dominated final practice at the Monaco Grand Prix.
Ferrari were one-two with the four-time world champion 0.345 seconds ahead of team-mate Kimi Raikkonen and Mercedes' Valtteri Bottas 0.435secs off in fourth.
Vettel's title rival Lewis Hamilton was only fifth, 0.835secs off the pace.
Jenson Button was 12th fastest on his one-off return to Formula 1 and has been hit by a 15-place grid penalty.
The 2009 world champion is standing in for Fernando Alonso while the Spaniard races at the Indianapolis 500 this weekend.
McLaren's engine partner Honda discovered after second practice on Thursday a problem with the MGU-H, the part of the hybrid system that recovers energy from the turbo.
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It and the turbo - which are one unit - had to be changed and as those are the fifth examples of those parts that car has used this season, penalties were inevitable.
It means Button, whose team-mate Stoffel Vandoorne was 10th fastest, will almost certainly start from the back of the grid, which means a difficult race on a track on which it is extremely difficult to overtake.
The session was interrupted towards the end by a crash by Force India's Esteban Ocon.
The Frenchman, driving at Monaco for the first time, clipped the inside barrier at the second Swimming Pool chicane, breaking his right front suspension and catapulting him into the barrier on the exit.
The car was removed from the impact-absorbing barrier within a few minutes and the session restarted for a final four minutes.
But although both Mercedes sent both Hamilton and Bottas out for a final lap, they could not close on the Ferraris.
Mercedes will have some thinking to do ahead of qualifying, which will be held under beautiful blue Cote d'Azur skies at 13:00 BST.
The world champions appear to be struggling around the streets of Monaco just as they were at the Russian Grand Prix two races ago.
Both circuits have similarities, in that they have low-grip, low-abrasion track surfaces. Coupled with the fact that the softest 'ultra-soft' tyre is durable enough to do an entire race distance around the principality, it is giving Mercedes difficulties in getting the Pirellis into the right operating window.
However, in Russia, Bottas managed to find a solution to qualify third, close behind the Ferraris, and jump them at the start before winning the race.
Mercedes will get a performance boost by turning their engine up for qualifying but at this stage it does not look as if the 0.2-second gain they get from that will be enough to allow them to challenge the Ferraris for the front row.
They may struggle to beat the Red Bulls, with Verstappen just 0.11secs behind Bottas and Daniel Ricciardo 0.162secs off Hamilton despite stopping on track at the end of the session with a failure of the brake-by-wire system.
The two Toro Rosso cars look set to contend for the positions in the top 10, after Carlos Sainz and Daniil Kvyat ended the session seventh and eighth, ahead of Haas driver Kevin Magnussen.