The 2009 world champion barely completed a lap during the first hour of running as he and McLaren practiced pitstops. But he was immediately quick after taking to the track and set his best lap of 1m23.510s on hard tyres shortly before 11:30am.

That was enough to knock Sergio Perez off the top spot by 0.310s, and with nobody else improving over the remaining time, the Sauber driver remained second thanks to the time set on medium tyres.

Sauber said that the Perez had earlier lapped on "prototype medium rubber", but Pirelli has since confirmed that those tyres, which are unlabelled, are of standard type. There are unlabelled tyres of each of Pirelli's four compounds in use at Barcelona.

Romain Grosjean spent almost an hour in the pits early on, Lotus having to change sensor equipment on his E20 after the Frenchman's initial runs. He gradually moved up the order throughout the morning and went third with half an hour to go.

Mercedes driver Nico Rosberg, who completed numerous stints of eight laps each, was fourth, just 0.001s slower than Grosjean while Mark Webber's Red Bull and Jean-Eric Vergne's Toro Rosso were next up, ahead of Force India driver Paul di Resta and Ferrari's Felipe Massa.

Vitaly Petrov, who was not scheduled to drive until Saturday, was a late call-up for Caterham after Heikki Kovalainen fell ill with food poisoning overnight.

The Russian completed a session-high 67 laps, the team confirming that it had made adjustments to the brake blanking and front wing flaps late in the morning. He was an encouraging 1.4s off Button's time and 0.7s up on Pastor Maldonado's Williams, which was slowest.

Morning times

Pos  Driver         Team            Time                Laps
 1.  Button         McLaren         1m23.510s           27
 2.  Perez          Sauber          1m23.820s  +0.310   57
 3.  Grosjean       Lotus           1m23.959s  +0.449   36
 4.  Rosberg        Mercedes        1m23.992s  +0.482   57
 5.  Webber         Red Bull        1m23.993s  +0.483   53
 6.  Vergne         Toro Rosso      1m24.216s  +0.706   61
 7.  Di Resta       Force India     1m24.305s  +0.795   48
 8.  Massa          Ferrari         1m24.318s  +0.808   45
 9.  Petrov         Caterham        1m24.876s  +1.366   67
10.  Maldonado      Williams        1m25.587s  +2.077   37

Autosport.com